I think that the 360/Wii will have games that sell more copies than the PS3 simply becuase they have bigger install bases, most people including posters here on this site, sees the PS3 seller only half of what the Wii/360 will sell each. So if the PS3 sells 9 million systems by the end of the year that means GTA4 has to sell to half of PS3 owners to sell 4.5, and does Europe even get GTA?
Biggest Games for the three:(all depending on release dates)
Wii: Mario, Brawl, and DQ(mostly in Japan)
PS3: Killzone and GTA4 (I don't MGS4 is going to be as big as everyone thinks)
360: Halo 3 and GTA4