Braska's Final Aeon. It was actually a really fun and challenging fight when I finally decided to face him without doing sidequests and powerlevel.

Zeromus would probably take second place. He was pretty damn tough, and probably the only reason why I didn't pick him instead is because he didn't Game Over me (came so damn close though).

Honorable mentions to Ultimecia, Necron, Orphan and the entire XIII-2 final boss gauntlet.