80's-super mario bros.
90's-final fantasy vii, viii, or ix i can't decide.
00's-shadow of the colossus.

70' To young to tell :) But I can go with pong
80' Super Mario Bros. ??? come on... I know this game is great but surely not best. What about tetris? I would say this is THE best game of that decade. Mario comes second.
90' Legend of Zelda: Ocarina of Time.
00' For now... World of Warcraft, but I doubt it will change.

80s: Donkey Kong
90s: X-COM: UFO Defense
00s: Portal

80s Super Mario Bros
90s Ocarina of time
00s Galaxy, so far...

Damn, Im not able to answer that question but if I had to choose... hmm, not easy, I feel like a betrayer to all the good games out there but... ok, here it comes:

80's Super Mario Bros.

90's Secret of Mana

60's - Had to do it, lol. Spacewar, I guess lol.
70's - What the hell, I'll get on the Pong parade. Space Invaders close second.
80's - What the hell are you gonna do, he was good once: Super Mario Bros.
90's - Can't decide between Sonic the Hedgehog, Pokemon R&B or Ocarina. Umm......Pokemon, too much fun.

00's - Either the Super Smash Bros. Melee, Kingdom Hearts series or Warcraft 3/WOW

80s. Super mario bros
90s. OOT or Goldeneye.
00s. so far Melee or Galaxy or GTA3

80s - Super Mario Bros.
90s - Street Fighter 2
00s so far - Guitar Hero series

Although I think its a little unfair to have a best of the decade game, since so many advances were done within on decade especially the 80s, a best of each console generation would be more deserving IMO.

80's - Super Mario Bros. 3
90's - The Legend Of Zelda: Ocarina Of Time
00's - The Legend Of Zelda: Twilight Princess

80's - Super Mario Bros.
90's - Final Fantasy VI
00's - Grand Theft Auto: San Andreas