My personal favorite is XX Slash.

Accent core, because it has a some improvements over its predecesors.

My most played is XX, hahaha I even got my PS2 repaired coz I busted it while playin XX. Well, latest one I love is XX Accent Core cause KY's new move is interesting to work on, although I don't really like the anime styled character image.

Accent Core undoubtedly.

XX. I've played them all (the ones that have all been released in the states, anyway) and the only one I've been fully satisfied with, both single player and vs, has been XX. Accent Core brought some great improvements, and it would of been my new favorite had they not opted to completely eliminate the story. That pretty much ruined the entire game for me, since I play fighters for the story mode also. Coming in at a close second would have to be XX Reload, which has been taking up most of my time lately.