I mainly want to know what games you think are worth getting but aren't on the PS2.

Best game on Xbox, by far, is Steel Battalion, anyone who says otherwise is a LIAR and you should punch them straight in the face.

For the GCN:
Tales of Symphonia
Phantasy Star Online Episodes I & II
Resident Evil Remake
Sonic Adventure DX
Sonic Adventure 2 Battle
Skies of Arcadia Legends
Final Fantasy: Crystal Chronicles
Resident Evil Zero

Ikaruga.

Rogue Squadron II and III.

On Xbox:
Star Wars: Knights of the Old Republic
Star Wars: Knights of the Old Republic II: Sith Lords
Jade Empire
Sid Meier's Pirates
The Elder Scrolls III: Morrowind
Ninja Gaiden
Dead or Alive 3

Gamecube:
Baten Kaitos
Baten Kaitos Origins
Tales of Symphonia
Skies of Arcadia Legends
Resident Evil Zero
Residen Evil Remake
Star Wars Rogue Squadron II: Rogue Leader
Star Wars Rogue Squadron III: Rebel Strike

On the gamecube,
Eternal Darkness, Pikmin, Super Mario Sunshine, Metroid Prime and Tales of Symphonia (soon to be available on steam) are the best games.

On the Xbox,
I enjoyed the PC version of Call of Cthulhu: Dark Corners of the Earth. I got it on GMG for $2 and liked it despite the need for a fan patch and a bunch of game crashing bugs. Xbox also Shenmue II.

The following website list a bunch of other decent game which you might not of heard of.
http://www.racketboy.com/guide/hidden-gems

Ninja Gaiden & Counter-strike on the original Xbox.

Some good GCN titles include Timesplitters 2, Tales of Symphonia, and Sonic Adventure 2 Battle.

Pretty sure Timesplitters 2 was on PS2. 

Yeah, my bad, I forgot it was a multiplat, lol.