I suspect there's already a thread like this floating around somewhere, but without a search function it's a pain in the ass to find, so...

Here are mine:

Vampire the Masquerade: Redemption
Disciples II: Dark Prophecy
Morrowind: GOTY
Toribash (derp)
Bioshock
Portal
Dragon Age: Origins
Mass Effect 2

Portal. Easily. I'm not much of a gamer, but portal really stands out to me because of the gameplay and the concept of it :>

But for the games that i do play, "geometry wars: galaxies" for the NDS, it's a crisp, clean shooter, similar to galaga, but you can move all over the place

and n+ for the NDS and computer.

Oh, damn. Thanks for reminding me. Portal too, of course.

Half Life 2.... man....

I'm getting so many recomendations for that Vampire the Masquerade: Redemption. I'm gonna be forced to play it eventually ;) Anyway, here are mine:
Morrowind GOTY
Soul Reaver
Toribash
Zelda: Ocarina of Time
Zeus: Master of Olympus (and extension)
Half Life 2

I think I covered most game genres there.

Older games:
Crash Bandicoot 1-3
Spyro 1-3
(These were some of the first games I played and they bring back great memories whenever I go back to play them again.)
Super Mario World (Best Mario Game ever)
Megaman Legends (I don't know why but I've always loved this game even though It's pretty hard to argue it's better than the 2D Megamans)
Ape Escape (Your a kid going around catching monkeys with a lightsaber, net,slingshot, RC car, etc. What more could you want?)

Newer games
Jak 1-3 (Best Playstation 2 series in my opinion. Lost frontier sucks though I didn't expect it to be good.)
Devil May Cry 1 and 3 great beat-em up games. (2 below average, 4 good but not amazing)

Dude u forgot the most ancient a fun game of a time ! Packman ^_^

TF2 is awesome, even though I suck at it, but I reckon Ratchet and Clank 1 and 2 are two of the best games I've ever played.
EDIT: I forgot Portal and Braid.
EDITEDIT: There's probably more, but I've forgotten. I might update this post.
EDITEDITEDIT: Klondike, on my I-pod. Pixel Legions.

My games:
Stranded II + mods (http://www.unrealsoftware.de/)
Toribash
Sauerbraten (http://sauerbraten.org/)
FIFA 10

Edit: oh and edna bricht aus (German)

Just Cause 2 for xbox of course