PS2: Tremendous selection of games (they’ve really turned me around since the originan launch). Some of the most practical, dependable controllers in the industry (I’d avoid anything with analog buttons, however). Fully supported. Backwards compatibility with PSX games and peripherals. Well-built system not prone to breakdowns.
X-Box: Good selection of games, but still behind PS2. Questionable durability at best. Space-waster. Some of the worst controllers I’ve ever seen, seemingly designed to “look good” rather than feel right in anyone’s hands. Future support, especially with lagging sales, in doubt.
Gamecube: Not all that great a selection of games. Development hampered by “family” reputation (not as bad as in the SNES days, but this will never completely go away) Controllers could use some improvement, and no decent joystick yet. Uncertain support (esp. regarding peripherals).
I rest my case.
(Oh, and this is just my opinion, of course; go ahead and buy the system you want. I’m not about to start another tiresome fanboyish war here.)