Deus Ex: Soooooooooooooooooo good. I adore this game. Probably my favorite game ever.
Half Life: Self explanatory. Completely changed my perception of PC gaming, and shooters in general.
Star Craft: Up until this game, I found RTS games boring. I was young!
Fallout 1 and 2: Very original setting, awesome gameplay, great characters, HUGE, immersive world.
Serious Sam: The magic of the original Doom taken to the extreme, and grafted to a hilarious story. I have very fond memories of competing with friends in trying to slay thousands of enemies with completely inappropriate weapons.