I'll pick them all because I have them all plus they all have games on them that you just can't get just for one machine. Plus I have three kids and it's just another thing to keep them quiet.
Gamecube = Sonic, Mario, Resident Evil, Zelda and Star Wars
PS-2 = GT3, Virtual Fighter, Kingdom Hearts and Silent Hill
X-Box = Project Gotham, Panzer Dragoon, Shenmue, JSRF and Mortal Kombat
PC = V8 Supercars with a force feedback wheel:), Morrowind, Age of Empires series plus a heap more and all the educational programs for the kids. Oh I almost forgot "Typing of the Dead" just like House of the Dead 2 but you use your keyboard instead, they put words on the zombies and you have to type the word to kill it great typing tutor.