Arcana Heart. The fighting system is easy to figure out, but rewarding to higher-level play. The multi-purpose Homing button adds another few layers to that, as well. Most of all, I love the customization inherent in the arcana system. AH2 was even better (more characters and arcanas, better balance and music, and the Homing and Arcana move buttons separated), but I haven't been able to play that for a year and a half...
Honorable mentions to Fate/Unlimited Codes, Melty Blood (spec. Act Cadenza version B), SSBB, and Guilty Gear XX Slash.