Metal Gear Solid is up there, definitely. So is StarCraft, and Brood War should be a given. SC just isn't complete without it.
I'm not really sure if I have a definite favorite game of all time. The games I want to play wildly vary, depending on my mood. Sometimes, I'll sit down and pour a solid hour into driving around the old stomping grounds of Grand Theft Auto 3 again to clear my head, or go and unlock everything in Soul Calibur while eating a supreme pizza and listening to rock and roll music.
It just depends on how I'm feeling at the moment that I sit down to videogame.