Would have said lineage 2 but they made so many changes that I felt like I was playing WOW, I would say L2 Chronicle 3 was the best.
Could have also said Generals but the expansion made everything terribly unbalanced, damn EA!
And maybe even SSB Melee but it was only really fun when I played with other people but unfortunatly tournaments don't come around as often especially now with Brawl out, even if you like it you can tell that game isn't that type for tournaments!