Tetra Master (FF9 card game). Though I find Triple Triad(FF8) to be extremely fun as well, I think Tetra Master is a lot deeper. Actually, when I bought FFXI like 7 years ago or however long it's been, you can actually play Tetra Master on it from the Playonline Viewer. I played that more than FFXI when it came out haha.
I like Blitz Ball, but it's just WAY too easy, and I think it gets boring quick. Though, maybe it's tougher at level 99, but the computer AI is pretty dumb for the most part. I'm only in the level 40s right now, if I ever get to 99 I'll let someone know if the difficulty changes haha.