Skies of Arkadia is a good RPG.
I don't like phantasy star online.
Grandia II is the best RPG I ever made on GC. (btw, it is released on PC (for sure) and on PS2 (I guess).

And for Kingof2v1, there is a 56K modem on the GC.
But I don't think that servers are remainning now, since the GC is dead...