If you're talking best modern game of all time, then I don't have a clue, and if I say, I will probably think of something else after. But I will go with The Orange Box If you mean best game (as in any) game of all time, I would say Sonic the Hedgehog 3 hands down.
At the very beginning gamerman said "the best game evr has 2 be zelda ocarina of time god that game rules it pretty much made most of the action/adventure games what they r today"
All of that I would agree with (besides the chat speak =P) That game has stuck in my mind ever since I've played it. But, that ones old, but my second favorite is too: Gotcha Force. Sounds funny, looks funny, but it is a great fast paced game.