This is an odd one to answer for me, so I'll post my top 5 fav fights.

Titan Extreme: It's one of the best fights to practice cast clipping, and is just generally fun.
Turn 3 of the Second Coil of Bahamut (Turn 8): A giant Target Dummy that has whimsical, yet fun mechanics...
Turn 2 of the Final Coil of Bahamut (Turn 11): Pretty much encompasses Titan Extremes Cast Clipping and Turn 8's Target Dummy freefire. Tethers are also really, really enjoyable.
Turn 4 of the Second Coil of Bahamut (Turn 9): A ton of mechanics and a ton of ways to optimize it. Love having to be aware for long amounts of time
Turn 5 of the Binding Coil of Bahamut (Turn, err, well, 5): Another fight that could be broken into multiple bosses, this boss teaches newer persons to build an awareness that later proves to become invaluable. Joining PFs is always fun–whether it be from groups who can't pass divebombs or making a few happy with clears, it's never a dull moment with PFing Turn 5 lmao