Deus Ex (PC)
This game really creates the illusion that every choice has a consequence. And I liked hacking every computer I could find, so I could read personal emails. Love it and can't wait for the high definition texture project to complete. ( http://www.offtopicproductions.com/hdtp/
Red Dead Redemption (XBOX360)
It's a game I always dreamed about. I love everything about it, excluding a few missions (namely in Mexico) that are pretty damn stupid.
Fallout 2 (PC)
It dominated my world for a long time and i've spent hundreds of hours playing this. Buggy as shit, but even that didn't bother me.
UFO: Enemy Unknown (or X-COM 1, whatever) (PC)
This game has everything. EVERYTHING. Propably my all time favorite game if I was forced to choose just one.
Silent Hill 2 (PS2)
Yeah, yeah. i'm a fanboy, shut the hell up. The plot is very moving and complex (for a videogame), the soundtrack kicks ass and I love all the characters to death. Also, it's the second creepiest game in the series. The first one was creepier, but the plot was too silly.
Fallout: New Vegas (PC)
With a few mods, it's pretty much a perfect game. Still waiting for the DLC's so I can start it all over again. Also, there are a few mods that are pretty much essential for an awesome game experience. Why anyone would play this on a console is beyond me.
Mega Man 2 (NES)
The only Mega Man besides MM3 I managed to beat.
This one takes the cake because of the most awesome NES music ever made and the pacing was just right. Not too long and not too short. The easiest MM game by far, but i'm one those losers who doesn't like to be too frustrated with the game difficulty.
Zombies Ate My Neighbours (SNES)
The second most fun co-op experience i've had. Everything about this game was too awesome for words. Full of thrills and chills.
Saints Row 2 (PC)
The most fun co-op experience i've ever had. The game engine bends to all kinds of crazy experiments. Tons of stuff to do and laugh out loud stupid characters / plot. Waiting eagerly for SR3.
Resident Evil 4 (GameCube)
The game looked so cool I bought GC, even though there was no other game that interested me in that system (well, maybe Eternal Darkness and Twin Snakes). Awesome and original level design, creepy enemies and most of all, extremely satisfying shooting with cool customizable weapons. Easily the best Resident Evil. Those who say otherwise are wrong.