But I always loved Catacomb 3-D, which was basically the original FPS. I need to play that one again. Oddly, Wolf3d didn't age nearly as well.

I play through a few levels of Quake now and then, but never get very far. I hate ogres (that makes me weird, I know), and the original maps overuse them, in my opinion.

I play through Half-Life routinely up until Xen. Ugh.

I had never played straight through Doom 2 without cheating--I started a "real play" last month. It's slow going, though... I find it a bit dull and have to be in the right mood.

Oddly enough, I often play my older console games. The Mega Man games, Metroid, Zelda. I guess those are more firmly linked to my childhood.

Oh, and Ultima VII. I load that up about once a month. 

Games I replay the original levels of:

Quake - lots and lots. Even these days, I still just jump in for a couple of levels.

Quake 2 - again easy to jump in and dish out some anti-strogg pain.

Unreal - I haven't really played individual levels, but I've replayed the game a few times.

And that is pretty much IT. Lots of others: Undying, RTCW, WOT, FAKK2, Blade, D3, HL2, Far Cry, etc etc, really don't inspire me to play again even if I really enjoyed them first time.

I guess for a game to be replayable to me it needs to be:

Very easy to get into any level.

Have the levels give enough supplies to start from scratch.

The levels NOT be too involved with a story and overall progression, i.e. they can stand in the own right.

The gameplay to be very fun.

...most games fall down on point 3 - i.e. the levels are too deeply entrenched in the overall game, and they don't feel right on their own, they don't feel like individual levels but as small, inseperable sections of the whole game. Most games are like that these days, only a few feel level-divided (e.g. Painkiller very notably but I don't fancy replaying that ;)). 

used to play THE Quake ep 1 and 3 just for the kicks, running thru in no time to relax my brain and forget the troubles of life (alternatively, kicking around some bots in q2/q3)

but not anymore

now I have like 5 or 6 modern games sitting on my hard drive begging me to play them :) 

Q1 & Q2 a few times. Actually just picked up the mission packs for the two recently and will probably do the same for them. As Shambler stated they're both quite well suited for a quick spin now and again....and still fun!

Unreal..replayed many times...imo one of the most immersive games made. 

I will once again state my opinion that games should not be so linear that they require you to start at level 1 and play all the way up to level 21 just so you can play your favorite. Too many games are like that today, and it's part of Shambler point about levels being too involved in the story; although the developers could still break the game into chapters and let the player choose which chapter to play after finishing the game.

Quake and Doom really nailed this point without making it too non-linear like some games, such as Enclave. In Enclave you could pick whichever level you wanted, but it didn't feel like there was any build-up or progression to get there--each level was its own experience, and it didn't feel like part of a greater experience when you go back to replay the game. Whereas in Quake, you have an excellent buildup from E1M1 to The House of Cthon.

And, of course, non-linear level design goes a long way, too. 

In the pursuit of more relaxed gaming sessions, I'm replaying old stuff that isn't as hard or intense as new stuff. I replayed the Zodiac mod and while it has very mixed mapping, it's a good mod and feeds the DX hunger I have.

I'm on to Redsun 2020, another large DX mod.

I hope the Deus Ex 2027 project gets finished. It looks really good. 

But there are some nice bits in there. CIA HQ was nicely involved as an episode. I was amused to find a familar arsonist ('have you seen my stapler?')in the New Mexico office complex.