Last generation I pretty much went Xbox for my 3rd party games for better graphics, acceptable controller, and the only online I wanted to deal with. For a lot of the generation, I preferred the PC to the Xbox as well, so that's where I played Splinter Cell and Chaos Theory.
GCN - 8/42 are on other consoles. All of those are available only on the PS2 and GCN (Def Jam Vendetta, Viewtiful Joe, Viewtiful Joe 2, Killer 7, RE4), had exclusive content (Pac-Man Vs / Pac-Man World 2), or games I bought before I had any other console (Tony Hawk 3, Extreme-G 3). I could see adding Super Monkey Ball 1 and 2, since the content is available on other consoles.
PS2 6/38 are on other consoles. 3 of those are GTAs, because I didn't want to wait / didn't know they were coming to other consoles. I re-bought San Andreas on PC. SSX3 is the only game I got for the PS2 specifically for the controller. Red Faction II was a gift and The Suffering cost $2.
Xbox 30/46 are on other consoles. Enough said.