Gamecube unless you want the XBOX as an HTPC. Even with my XBOX currently connected, I'd rather have an HTPC.
My XBOX has gotten 0% of the playtime despite having component cables for all consoles and it being the only true-HD one out of the bunch (I don't consider XBOX Media Player and XSNES9X and other HTPC-capable software running on an XBOX to be "XBOX Playtime"). It's all about gameplay and that's all exclusively on the Gamecube (Not as biased as it sounds ).
Heck, even my N64 is getting infinitely more playtime than my XBOX... People have been coming over nightly for Mario Kart 64 and Goldeneye 007. I guess they've had a resurgence in popularity! I showed my friends Mario 64 & Mario Kart 64 running on my Gamecube using the ripped N64 emulator (From the Zelda bonus disc) and they had me break out the original. Been almost nightly ever since (Though I enjoy Super Mario Kart more and have tried enticing them for JUST ONE round of four-player Mario Kart Super Circuit).
I just got Steel Battalion for XBOX so at least *I* intend to give it a little more playtime but other than that I always just wait for the PC ports (Splinter Cell, Halo, etc).