-the first fiend in Quake: e1m2, when the boxes float up in the air and you're thinking what's this weird magic shit going on here, the suddenly WHAM!

-the huge beach level in Far Cry (same one as in the demo). a really great overall experience of sneaking around and climbing up the island, the osprey at the top was epic too.

-having your head eaten in the Doom3 demo

-First seeing the Hell Knights in Doom, at the end of Doom episode 1 (shareware) - hearing that scream in the middle of that giant pentagram was a huge oh shit moment.

-Using a dwarf for the first time in Myth: The Fallen Lords... so awesome blowing the shit out of tons of guys and having their body parts fly around tracing blood in a then-unprecedented manner. 

The first zombie in Resident Evil. You know the one. This and the hellhound smashing through the window.

The first time the sirens come on in Silent Hill. You know those sirens.

Almost every time I did something new in Duke3D. The helicopter crash in level 1, the shrink ray, freeze ray, devastator, the first time the goo things climb your face, all of it was awesome and very innovative at the time.

Cthon rising out of the lava. Did not see that coming.

Nuking that one town in Fallout 3. And the nuke in CoD4 blew me away too - figuratively.

Hmm why is it so hard to think of these? I know I've been blown away so many times in games, hard to remember specific ones though. Maybe I'm jaded. 

Venturing into the East in Morrowwind. Seeing the spiraling fungoid cities for the first time. Speaking to an encampment of wild elves who were challenging the ruler of the city. I felt like I was in the middle of an old dying earth style fantasy novel. 

Blacksite: the helicopter fight with the tentacle monster on the bridge

MOH Airborne: every time you jump out of the plane!

Prey: the shrunken planet

Call of Cthulhu: the tidal wave

King Kong: climbing up the mountain

Farcry: the level with the little wooden bridge

In fact, most of Crysis is awesome moments. 

UT3: the spaceship level is quite awesome 

Super metroid ending, beginning of Zelda III made perhaps stronger by first playing it in a store before the release of the SNES and then anticipating it for months before getting to play it at home.

Playing stuff on LAN with friends.

Discovering Doom/Quake mapping/modding. 

1) Seeing the shaky-cam footage of half-life 2 at e3 for the first time. Serious nerd chills! I remember starring slack-jawed at the screen seeing what they were doing with physics and characters.

2) That shaky-cam footage of doom 3 from the nvidia geforce 3 presentation. It looked like pre-rendered footage! And then seeing it moving... THE bathroom scene with the pinky demon will stay with me until I die I think :)

3) My first ever online Quake deathmatch. Holy fuck that dude is another real person, shooting me in the face! I better move!

4) In Freespace when that giant enemy capital ship appears for the first time (the lucifer I think it was called). That game created such a fantastic sense of scale with the ships and seeing that impossibly large ship come out of the wormhole and annihilate my friendly capital ship was amazing. 

- The rising of giant tentacle monster in half-life

- The hang glider flight in Far Cry

- Watching Quake Done Quick the first time

- Alien versus Predator (the first one) when you realise you can crawl on wall and ceiling playing alien campaign

- Getting out of bunker 13 and seeing the wasteland the very first time in Fallout

- Using the first wind tunnel in Quake

- The gravity gun in half-life 2 

Not really in the order but...

- First fight with a CyberDemon in Doom2
- First CacoDemon killed with a chainsaw in Doom 2 (so funny)
- First fight with a Shambler in Quake
- Beginning of Normandy Overlord operation in COD2 (as realistic as Saving Private Ryan)
- HL2 cut-scene before entering the latest level (architecturally awesome)
- Doom3 Hell level ambiance (so scary and exactly as I imagined it)
- Doom3 Mancubus cut-scene.
- Quake 4 Network Guardian (Flying boss), and Stroggification level

... and for sure many others I don't have in mind right now.. 

First level of God of War 2. Pretty much the whole thing.

Sunrise on the first level of Crysis.

Seeing the weird geometry of Quake 3 patches.

Too many in Half-Life and Half-Life 2 to mention, but quick picks would be Surface Tension and Highway 17.

Playing Wipeout 2097 for the first time.

The skyboxes in a lot of Unreal games. Special mention to the Phobos levels from UT/ut2k3 and UT3.

Playing Doom and Quake for the first time. 

Ultima VII - getting out of trinsic and seeing on my map how huge of a world there was to explore. also, the fact that it came with a cloth map was wicked!

Doom - first time playing, the e1m1 music, also first time firing the shotgun. Sadly, I don't remember playing quake for the first time. :-(

Full Throttle - the intro cutscene, with ben jumping over the hovercar on his bike and smashing the hood ornament, is great :D

Half-life 2 - yeah, too many to mention - but exiting the train station and stepping out into the city square was awesome - it felt more photorealistic than anything I had played before. 

- Doom in a map (I can't for the life of me remember which one) when I first heard the crying baby sound effect. By far the creepiest moment in my early PC gaming days.

- Ratchet & Clank, the first time I used the Rhino, awesome weapon. Same goes for the Hammer in UT2003.

- Strangers Wrath, too many moments to call out just one though Mongo Valley stands out.

- Burnout 3 Takedown, the first time I played the takedown mode.

- Tomb Raider, one of the first temples, it's lower level was flooded, you have to jump and climb a bit, you reach something like the third tier and that memorable music with the mostly male choir starts up.

- UT CTF, too many memorable moments to list. We spent more time than I care to remember organizing lan parties, online matches, etc... This game had the very best bot play, sniper rifle and then camera the guided mode of the Redeemer. Modern multiplayer can't touch it.

- Q2 dm, nothing compares.

- The first time I came upon Zombies in Quake.."Oh crap! The double barrel ain't killing 'em! Oh crap, oh crap!

- Just Cause 2, grappling to a helicopter, take out half a dozen enemy copters, eventually get shot up too badly to stay in the air, jump out and open the chute, grapple the copter that shot me down, rinse and repeat. This game gives the player and awesome sense of power. Dismounting the turrets and taking down an entire power plant, platoons of baddies and all sorts of military hardware...bliss. 

maybe I am easily impressed.

Lets start with Unreal.

I think most would agree about walking out of the spaceship in Vortex Rikers. One of my other favorites was The Sunspire level, the sense of scale at that time was amazing (even at 10 FPS on my shitty comp at the time). Shambler has already mentioned Bluff Eversmoking, that was a crazy level for the time. 

Amazing what sound effects can do, considering that HL2's biggest rival was doom3, which had blatently better graphics engine, but nowhere near as much scope.... 

- amnesia the dark descent

- thief 1/2 the gameplay experience

- gothic2 , meeting up with old guys from previous game

- oblivion itself 

i found morrowind more awe inspiring than oblivion, myself.

i remember the first time i set out from balmora to get to ald'ruhn on my own. it really felt like this epic adventure, especially near the end when my weapon and armor was getting pretty broken and i was even worried if i could get there alive.

and then there was another 'awesome moment' right after when i was looking at the world map and i realized i had only walked around about 1/8 of the entire map.

now THAT was awesome. 

Ummm...

Laugh all ya' want but the opening bit in Toy Story 3. 

THOR - first real moving 3d game, h� something rolls out of my monitor.
WOLFSTEIN - the cubes getting smaller.
DOOM - state of disbelieve and anger.
TOMBRAID - wauw levels even larger.
DUKEN3D - becoming drunk underwatercolors, playing pool, what a cry.
TUROK - levels hardly without end.
THE GIFT - funny gnome keeps me mad.
QUAKE - walking in surprise around an ogre, he's round! Chtongs door, Wind tunnels.
UNREAL - three months I thought I lived on mars.
RUNE - fine epic story, what an axe.
SUBCULTURE - still searching.
ABES WORLD - unique playground.
HALFLIFE - the size of the metro, and the game.
QUAKE2 - improved game and apearance.
QUAKE3 - winner.
QUAKE4 - too much to explain, the strawlberry wiches. 

- quake was my first pc game. Just getting to the end of e1m1 gave me a great feeling of achievement. We have played without mouse and strafe keys. Can you imagine the fear we had running from dogs? :)
- first meeting with an ogre (grenade in the face of course), fiend and shambler
- everytime I can win a close match in quake 3
- everytime I manage to beat a quake speedrun record (after countless hours and tries)
- laughing to the humour in the witcher
- when I manage to solve the more difficult puzzles in games like portal or spacechem
- the whole escape part in portal
- wathing quake done quick for the first time (as someone mentioned above) 

First run through The Dismal Oubliette. I was playing with a controller at the time, and man was it hard. I'll never forget the sheer build up of intensity in that map.

Some wonderful moments in the original Tomb Raider, positively dripping with atmosphere, although the second half of the game was something of a let down. I'd rank the second episode (St Francis Folly, Palace Midas, The Cistern, The Colloseum, etc) as the best, but the first map was the first time I'd ever seen anything like that and my jaw was dropping.

The awesome opening flyby movie in the original Unreal. Exiting Vortex Rikers too, as was said above.

Unleashing Knights Of The Round for the first time in Final Fantasy 7; that one rocked and I think I must have laughed my head off at how stupidly overblown it was. Some nice little nods to some of my favourite music in that game too: "Loveless", "Sister Ray".

The dogs in Resident Evil, of course. Another from upthread.

Lots of great moments in Quake II. I loved the "dropping the Berserker into lava" bit, my first fight against a Tank, the Outer Courts map, finding all the cool Easter Eggs.

Neverwinter Nights was good fun too and sucked up a lot of my time (must be due a replay soon). Best set of metal/lava maps since Quake.

Getting Doom 3 running at maybe 25 FPS on my puny GeForce FX 5200 (man, was buying that a mistake or what?), there were some beautiful scenes in these but the whole design aesthetic was awesome. Every room had something moving in it, something active, this was a world that lived.

Seeing RMQ's take on e1m2 was an astonishing moment that's probably not appropriate for this thread, but still quite flabbergasting all the same.

Rage is currently providing some good ones. I think the Dead City is going to be one of those truly memorable maps, but right now the Big Fucking Grin moment was the first time a Wasted shouted "WANKAH!" at me. 

UNREAL - three months I thought I lived on mars.

Okay, after Unreal, my Quake moments :

- the first fiend, for the reasons Tronyn mentions

- the first Shambler for obvious reasons

- e1m3 because it made very good and obvious use of the 3d that Quake brought.

- the opening of e1m5 was amazing at the time

- the first time I fired up retail Quake with the music. The music, and its fitting in with the game, still blows my mind but at the time it was a big change from the usual sythesised midis. 

funny... thinking back on quake. i don't really recall being awed by it in any specific sense. maybe on some odd and novel things like, for example, the floating water pool in the start map.
i just remember being awed by the game in general. the whole full 3d stuff and whatnot. 

Serious Sam: The First Encounter!

Never before have I seen any creature in game so large, so seemingly invincible! I was like "Holy crap! WTF! That was huge!" 

Priveteer II: the darkening - the story and cinematics in the game immersed me very much.
that was the first game i played properly. i liked the atmosphere of the worlds i visited, and i was totally in love
with main actor playing main role - rather young Clive Owen ;)

Doom II: Was my first fps game i played. Main point that turned me against this game at first was rather vulgar violence. I was disgusted by the blood particles dropping from the mobs when I shot them. That felt utterly evil and disgusting. But after swallowing that, I became obsessed to see what's behind the next corner as I proceeded. That's the main thing that kept me interested in FPS games: to see what's next.

Quake: Well, I was totally blown away by atmosphere, sounds and 3d. I liked evil stygian style of the game. First time Ii played through the game I used only keyboard... Can you imagine? I looked up and down using PgUp PgDown keys... It felt ok for me, I got used to that :) Also, I played at 320x200 res. But then, I got internet and started getting hints on how actually it must be played and forced myself to use +mlook command. And after I beat game for the second time on harder setting with better resolution (got proper gfx card), I was totally friendly with mouse gameplay. And of course with internet available I go the idea that it's possible ot create your own worlds. You know the rest of the story ;)

Unreal: I was totally blown away by the epicness of this game. The most remembered moment of the game was then I first exited from the Rikers prison ship out to the wild nature and sun. I also was blown away by the music change to some epic ambient. I'll never forget that moment, I nearly got tears of joy. I ran around, looked at the sun, at that huge cliff on the left from the ship's exit, and at that giant waterfall... It all felt unreal :) Further on the game I of course was totally impressed by Bluff Eversmoking level and by the dead ship level, with total darkness and eerie atmosphere of the ship. Music in unreal was and is one of the best.

Rune: I was impressed by the gamedesign, the fact that u can grab any weapon, part of the corpse and fight with it. And you had to eat lizards to replenish hp. And i liked how big and epic was the game, from the caves you went further to Hel underground, then further to that snowy castle, then Ragnar changed its form to bigger uber warrior. All this accompanied with awesome sounds, level design. One of the best and brutal games ever, I'm replaying it right now, and it still rocks.

Half-Life: I was scared by the events in that test chamber so hard that my heart pounded as it was of the main hero. :) Also, I liked the theme they used in the game: cold war era is the best time for fps game design i think. Oh and of course i was totally impressed by Xen.

Quake 2: Most of all I liked the ambient sounds in the game. Proper industrial alien atmosphere there.

Alice1: The room with broken floor, walls. Where there were flying pianos and shit in the skybox. It was too werd for that time and I was impressed. Entire game was totally weird and with nice designs, there're alot of moments I liked.

Maybe I'll remember something else (there definitely are more games with good moments), will post later. 

Coloured lighting with no 3DFX card!! WTF?!?

Also the scene in Half Life where you have your first encounter with the soldiers. Just after you see one of them execute a scientist, and it all 'kicks off'......