I play online games on my PC, and other games on my Wii/XBox/PS2. I find that games I have on all of them are fun
PC: ... KOTOR 2: The Sith Lords (Don't have many other games for it anyways)
XBox: ... Tough... Either KOTOR 1 or Fable: The Lost Chapters
PS2: Rock Band. Only game I really have since we got it earlier in the year...
Wii: Either Metroid Prime 3: Corruption, or Guitar Hero 3: Legends of Rock.