Right now i play d2 (pc) and god of war (ps2) but i am waiting for starcraft 2 and the new battlenet (2.0) which will be released end of the year ( at least i hope so)
on a console it would be Legend of Zelda: Ocarina of Time (but in general all zeldas above the rest)
for the PC it would be Total Annihilation hands down
Actually I'm going old school on this one and say space invaders. I got the high score in all the arcades back home and still play it occasionally when I have time.