Well, each console kinda have one interesting unique feature:
XBox:
+4 Players with no multi tap.
+Custom Soundtrack ability: nice to listen to "Daikenkai", while capping people in
GTA: San ******s.

+Better online: Live
+Comes with hard drive.
-Controllers maybe a little unfriendly with some arcade game, like fighters.
-DDR game on that kinda has alittle bad timing.
-DVD player requires a remote.
-Live isn't free.
PS2:
+Best controller
+Support non online 8 Player games through 2 multi taps.
+Bemani friendly
+DVD player: No Remote required.
+Free online
+Can play PS1 games
-Slow online
-Doesn't come with Hard Drive. Also, slim model doesn't support it.
-Needs multi tap for 4 Players. Well one multi tap gives you 5.
Gamecube
+4 Players with no multi tap
+Best 1st Party titles
-No online
-Not a DVD player
-Some games are 2 disks: ie
RE4 I think.
-Controller seems only useful for platformers and FPS.
But to me, the graphics on all three of them are the same. But I use the PS2 and XBox more than the GC, since those are the only two I own so far.
But Wii will be the next console I'll be getting, which will also mean that I'll own a GC soon.