Truth be told, all 3 Systems are worth owning. PS2 has the incredible library of hits , XBox has some treasures, and the Cube even has quite a few that should be owned by all.
I prefer my PS2 over the rest, probably for the games I like. Metal Gear Solid Series (Though gotta get Twin Snakes on Cube), Zone Of The Enders Series, Ace Combat Series, Virtua Fighter, SOCOM 1 & II online no less, RPG's galore...PS2 has all the big name franchises. Oh yeah, and there's this pretty cool Macross game...not a system seller but, cool nonetheless, you know if you're like, maybe a Macross fan.
But, a Cube is $99. Viewtiful Joe, Resident Evil, Rogue Squadron, Metroid Prime, Zelda, Soul Calibur II (Link rules over the other lame Heihachi and Spawn characters), Metal Gear Solid: Twin Snakes, and the best version of Sonic Heroes are all great reasons to get one too.
XBox: Halo. Halo 2. Panzer Dragoon. Pretty good Star Wars games. My least fave system but, still a lot to offer.