Xbox - Oof, this is a toughie... Probably Halo 3, but at the moment it'd be Borderlands 2.
Wii - I probably spent the most time with Mario Kart Wii or Animal Crossing: City Folk
3DS - I suppose it'd be Mario Kart 7, don't have a wide variety of games on there, yet.
DS - It's a toss up between Dragon Quest IX or Animal Crossing: Wild World
Xbox: Probably the Skate series or Halo, I don't use my xbox that much
Wii: Mario Kart Wii
3DS: Mario Kart 7, only game I have because I just got a 3DS this week
DS: ACWW, So much time spent on that game!
Xbox360: Assassin's Creed III (Coming October 30, 2012)
3DS: Animal Crossing: Jumpout (Getting the Japan version on November 8th. Soon will have the NA version)
DS: ACWW.
Wii: I don't own a Wii personally, my sisters do and AC:CF is a really good game
Well, I don't own an Xbox and this thread needs more PS3.
PS3: Little Big Planet or Infamous/Infamous 2
Wii: Either Skyward Sword or Kirby's Return to Dreamland
3DS: Kid Icarus Uprising
DS: The World End with You
Xbox: Any of the COD series and Alan Wake (I couldn't choose, lol).
DS: Animal Crossing: Wild World
3DS: I haven't actually played anything on the 3DS.
Wii: WarioWare: Smooth Moves.
Xbox: Halo 4
Wii: City Folk or Skyward sword
DS: Pokemon Pearl or Spirit Tracks
3DS: Have yet to own one, but I'll definitely pick one up for AC: New Leaf
Xbox: Assassin's Creed III.
DS: Summon Night: Twin Age or The World Ends With You.
3DS: I think it'll either be AC: New Leaf or Fire Emblem: Awakening.
Wii: I don't like a lot of Wii games... I suppose Mario Kart Wii is fun. c: