I can tell you my experience with the few games on the list I've actually played...

Half-Life: its worth a look if you can get it cheap (but despite being so old it seems the price is still outrageous for a new copy of HL, at least in my local stores). I know a few people are going to have a dummy spit over this but I don't see what the big deal is with HL... though to be fair I played it fairly recently, long after the engine was outdated etc. I found it to be reasonably enjoyable and the levels were ok, but I made the mistake of playing it on HARD (which was actually pretty tough with the marines) and I gave up on one level where I just kept getting my ass kicked. I'll get back to it one day... or I'll just get HL2.

RTCW was a pretty solid game, but also fairly uninspired. For me, it was worth the play because I enjoyed checking out the maps, but the game itself wasn't terribly innovative or enjoyable. I almost finished it, but got very frustrated with one of the later maps, so I gave up on it.

BF1942 is a load of wank, the engine sucks (player control feels awful, at least while running around as a man). Game concept was good on paper but just sucked for me when I actually played it (running or driving or whatever around gigantic maps trying to find people to kill and shooting pixels in the distance), not my kind of thing. As far as I know its mostly multiplayer oriented too (is there any single player? If there is its probably just against bots?)

Max Payne I thought was really well done, I'd definitely recommend that game (and I hear Max Payne #2 is even better, though I've yet to play it myself). Everything was polished and the gameplay was fun. The level of difficulty (supposedly self-adjusting to suit the player's skill, dunno if I believe that though) was good, pretty well balanced game. I played through the whole game in a couple of days solid play, which might not say a lot for its playtime but it does show I was enjoying it a lot (or I had nothing else to do at the time!)

Call of Duty is an excellent single player game and I'd recommend it to most. Its no secret that I'm not terribly fond of single player FPS games (as you'll see above I didn't even finish the first couple I mentioned) but I do get into them when they're as well done as Call of Duty was. Nothing terribly revolutionary, but everything was well implemented and polished.. the gameplay was a bit varied too (different scenarios and missions, sometimes driving vehicles or manning AA guns, etc). The gameplay though was extremely linear and you are never really offered any choice as to what to do or where to go, which may annoy some people... personally I enjoyed it this way as there was never any confusion about what to do or where to go, and whilst the linearity is a little limiting for the player it did allow the designers to control the gameplay very well (with regards to setting up battles and placing health etc etc). Levels were very nice and they got a lot out of the old Q3 engine!

From what little of the multiplayer in Call of Duty I played, I can't say I enjoyed it very much... character animations didn't look the best for some reason, levels seemed to be mostly modified single player levels and weren't so fantastic for multiplayer. Weapons didn't feel too good in multiplayer.. I dunno, bit ho-hum. 

Half-Life
Return To Castle Wolfenstein
Undying
Deus Ex 1

and maybe:

System Shock 2 if you like ass-ugly graphics but a good game.
Morrowind if you have no life to spare.

don't get:

Any lame and tedious real-life / WW2 recreation online game *yawn*.

Any vastly overrated outdated pointless shooter.

Any crappy console conversion. 

get

Half-Life
System Shock 2
Max Payne 1 + 2
Deus Ex (1)
Far Cry
PainKiller (out soon)

RTCW is over rated, nice maps, crap gameplay.
soldier of fortune blows, both of them.
I finished project IGI 1, although being a realistic shooter I can draw many comparisons to Far Cry. You ahve to get from A to B, but how you go about this is completely up to you, graphics a bit dated now though.
Blood 2 loller, pants. Get Blood 1! 

...I'll disagree with Frib here, and say BF1942 is pure heroin if you play it on a lan with a bunch of mates. Single player is wank, and online multiplayer is just counterstrike with plane camping. 

I liked and recommend :

HL and opposing force
rtcw for the maps
max payne
MOH:AA for some cool levels/sequences
NOLF 1 & 2 are great, well worth getting
SOF1 is reasonably fun but nothing original, get it only if its cheap
Undying

I dont recommend :

SOF2 cos it sucks.

Also suggest :

ALice
Rune
Elite Force 2 (kickass maps, and very well poslihed.)
AvP 2 for the marine misions
Unreal
Wheel of TIme 

DKT! 

If you buy Breakthrough, i get money. (maybe) 

....kicks serious ass. Gameplay is good (even though there are really only two character types that can win the game easily) and it's all about the atmosphere. 

Get:

Half-Life
Max Payne (and 2)
NOLF 1&2
Undying
System Shock 2
Deus Ex 1 (and possibly 2)

Skip:

Return to Castle Wolfenstein (IMO, truly nothing special. If you want average, banal games, there are lots of others to get.)
Blood 2

I haven't played:

The rest

Also get:

Enclave (for great levels and art)
GTA3 and/or Vice City
Serious Sam (assuming you find it for < $20)

Other possible candidates:

Alice (fun and cool for a while, but gameplay has little depth and the last few levels are frustrating.)
FAKK2
Duke Nukem: Manhattan Project (if you think side scrollers are cool, and you find it for < $20.) 

For mods:

Half-Quake (for Half-Life. Vondur says it's pretty good.)
System Shock 2: Rebirth (for System Shock 2, obviously. Not really a mod, but it has some really cool high-res models that replace the originals. Unfortunately, the pack is not complete and the guy working on it apparently has gone pro.)
I wish I could recommend mods for Alice, but they never released an SDK for it. ;(

Also, I'm sure nitin can recommend some top-quality Half-Life mods. 

... anyone who tested a demo of Doom3 ??? It seems to be really a good FPS, related to screenshots and trailers I've seen on http://www.doom3.com 

i really loved call of duty SP, esp. the russian missions. nothing too ground-breaking, but i really enjoyed the fact that it tries to bring you in as one soldier in a large front instead of the rambo-esque lone soldier against everything. I finally got cable internet last week and CoD being the only game I've installed/played since i reformatted recently, i played a little MP. the multiplayer is a total campfest, although i did have a few fun games of team dm. i'd still recommend it if you're not tired of WW2-era and like SP.

then again, i fall down alot. 

I took another glance at Doom3.com; the screenies are showing an exceptional level of polish now -- wow 

No, no-one's heard of Doom3.

Jeez, lame troll but kinda funny nevertheless. 

can't play ball, can't play ball... 

System Shock 2 is well worth seeking out (I got mine for NZ$5 from Cash Converters.) Sure, the models look poor, but the story and atmosphere are incredible. Known bugs: Some regions of the ship seem to spawn more monsters than elsewhere (e.g. the corridor on level 5 leading to the tram - there always seem to be two arachnids, one assassin, a midwife and kamikaze protocol droid hanging around. Not fun when you need a recharge.)

Half Life: There's a "Game of the Year" edition out with OpForce, CounterStrike and Team Fortress bundled with it. Try getting Poke646 as well if you're going with HL, it's well worth the download. 

That Wrath, what a kidder 

if the low quality models mean that much to you, there's an ongoing project to make hires versions of all ingame models... it's been inactive for a while now, and not all monsters are replaced, but i definatly recommend it if you pick it up.
search for "system shock rebirth"... 

we kicked their asses in the second half! 

I only support England if they're doing well. I umm... can't remember when that last was. 

I'm with wrath on this one. Not that I care anyway (I loathe most sports). 

tsk tsk, a fair weather friend . . . where will you be if Nazis (this time, Undead Nazis!) start knocking on their door?

But I understand the sentiment -- like in the Final Four, which ever team from my state gets there, even though I'm a Chapel Hill Alum, gets my support.

NCState was robbed! 

Chapel Hill alumnus? Cool. I was listening to the radio today and I found it quite amusing to hear all the Chapel Hillians and UNC folks say they were going to cheer for Georgia Tech instead of Duke, just because of the intense rivalry between UNC and Duke. Apparently you're not like that, though, so I can't laugh at you. 

Well, I have already played: Half-Life 1, Soldier of Fortune 1, Max Payne (on PS2), Unreal (but didn't have a chance to get too far, probably worth revisiting)...

thanks for all the insight, Call of Duty seems like a sure bet. 

... and LOL half hour later when I thought about it again.

I needed that.

http://www.spawnpoint.org/wads/scampie_test.txt