Gamecube. I had access to a friend's Xbox for several months and I played the shit out of Halo, but other than that the Gamecube was pretty much the only game in town for me as far as home consoles. But StarCraft and Diablo II ate RIDICULOUS amounts of my time.
As far as being a fanboy goes, I don't know. I didn't have any animosity for the other platforms, I just ignored them. I actually kind of surprised myself, because I had expected to get a PS2 for Final Fantasy, but I never got around to it. I just didn't seem to care. (I still played a lot of PS1 games, though, including FF Tactics, so that may have been part of it. And FFX didn't attract me like all the previous games had, for some reason.)
Now I'm catching up on PS2 games thanks to full BC on my 60 gig PS3, and I've got Panzer Dragoon Orta, teh Haloz, and a few other Xbox games. (And by "catch up" I mean buy them and plan to play them but actually don't.
)