In no particular order:
Resident Evil 4: omg fun. Ignoring how it made Resident Evil more fun (for me, other people understandably like the other games playstyles better), it's just downright fun. Replaying it for the supa weapons doesn't hurt, either.
Dead Space: It's just so fucking
good. And Isaac Clarke has one of my favorite designs ever. UNF.
Half Life 2 (Including the episodes): Worth playing through at
least once a year.
Team Fortress 2: The only game I've logged over 200 hours in I think, if not more. I wish I had the updates, though
. Also Xbox live.
Rez: So much fun just to watch. Also surprisingly epic at the end for a game abouWHITE NOISE IS GOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOLD
Bioshock: Same as Dead Space. So good, Big Daddies have an
AWESOME design, and that voice work is just fucking
awesome. Also pretty unnerving.
Strangers Wrath: I'm a sucker for westerns. If it had some kind of replay value it would've probably been my perfect game.
Donkey Kong Country (1, 2 and 3, but not 64): Fucking nostalgia. I also wish I could be playing these right now. All at once.
Earthworm Jim (series, even 3D): Literally my most favorite childhood character ever. I once woke my parents up at
7:30 AM Just to watch a VHS of it that I rented the night before on their TV/ VCR.
Megaman X: All of them. I fucking love Megaman.
Special Mention for Super Mario World, Left 4 Dead, Megaman Battle Network and Sonic the Hedgehog 2.
Double Special Mention For Psychonauts, Splosion Man and Borderlands.