Whoever said Halo is nuts--Halo is a ton of fun, but it has plenty of issues, not the least of which is the story it borrowed from HalfLife, which itself falls short of a title such as this thanks to it's idiotic ending sequences.
For me:
Console: Alex Kidd in Miracle World -- better than Mario in nearly every aspect--goofy, quirky, platforming fun
PC: Q3A -- I hate to give a nod like this to Q3A, a game that has no story, no particular levels, etc--however, this is really a look at gaming in the future..i've been playing it for years now. Q3A is a game that is so solid in its foundations, it has endless playability. It never gets old, it never feels worn. It took me about six months to warm up to it, but when I did, the collision detection, mechanics, and balance of the game are so spot on--I can't think of a better experience I've had on the PC.
Honorable Mention: GPL (for the same reason as Q3A--it is THE racing sim, to this day)