If you mean best as in what moved the most hardware, it's difficult to go against Super Mario Brothers for the NES or Tetris for the Gameboy. Both of these games ultimately moved more than 30 million software units when bundled with their respective hardware, and virtually created new industries that hadn't previously existed. While it's too early to tell at this point, I believe Wii Sports may be in the same category eventually; it's closing in on 10 million sold already, and may be as high as 15 million by the end of 2007.
If you mean best as in which games were most technically impressive, I'd have to say Super Mario 64. There really was no game even remotely like it when it came out, and it would not be a stretch to say that I bought the N64 just to play that game. (I'm sure I wasn't the only one either.)
By comparison, none of the Playstations has ever launched with anything particularly revolutionary. Halo for the XBox brought some new elements to consoles, but again didn't do anything that hadn't already been tried on the PC before. Its main achievement was bringing the online shooter to a much greater audience, and making it more accessible than ever before. The early Genesis/Megadrive games looked amazing compared to the NES, but they were pretty simplistic; I don't think they quite qualify for this category either. Maybe something on an obscure platform like Neo-Geo would work here, but I'm sticking with my answer of Mario 64.
End of 2008 totals: Wii 42m, 360 24m, PS3 18.5m (made Jan. 4, 2008)
