My Data fight rankings: (I think all of them are great to be honest, so this is really just ranking which ones I like more than the others)
13. Saix - Great fight. I think his DM lasts a little too long, but other than that, I don't mind him too much.
12. Luxord - Massive improvement over the KH2FM incarnation. This one I can actually see myself fighting for fun.
11. Larxene - Better than the KH2FM incarnation for sure. The DM is hectic, but manageable.
10. Xigbar - I like this about as much as the KH2FM version. The reaction command is sick as hell, too.
9. Dark Riku - This fight can take a while to wrap ypur head around, but it really is a good example of controlled chaos in boss design. That DM is satisfying to navigate, too.
8. Marluxia - Really, really fun fight. Great use of unblockable attacks, and the spin on the few returning attacks from KH2FM are appreciated. I love fire spells being able to instantly destroy his armor.
7. Vanitas - Pretty awesome fight. I love that you can just flat out nope.jpg his DM.
6. Young Xehanort - Makes for a fantastic warm up for the other fights while still being a threat in his own right. Osaka Team's come a long way from Mysterious Figure.
5. Terra-Xehanort - Really fun fight, and I like how his DM runs counter to literally every other one in the game; the less mobile you are, the better.
4. Master Xehanort - I personally consider him the true final boss of the game (with the secret boss being the ultimate superboss above that) He can be kind of overwhelming if he knocks you off tilt (and that DM, holy crap) but he's pretty fair through it all, and really makes you work for those openings.
3. Xion - I feel like Xion kinda falls into that controlled chaos category I mentioned in regards to Riku earlier where the fight itself looks a lot scarier than it actually is. Nonetheless, she definitely keeps you on your toes (especially if you want those Risk Dodge openings), and makes for a worthy successor to Data Roxas in KH2FM.
2. Ansem - I love that giving the Guardian to Terranort left them free to go balls out with projectiles and traps for Ansem's moveset. His DM phase is a joy to survive (and I appreciate it giving underused accessories like the various Rosette equips some use as well).
1. Xemnas - As good as this entire bunch is, Data Xemnas is just on another level entirely. It's perfectly paced. It makes fantastic use of Sora's toolkit (Be it all the fun ways to shut down his multiple shields OR using them to flowmotion in for an opening). And all of this is topped off by him having hands down, the best DM in the entire series. There isn't a single DM in the series that makes you feel as badass when you block the entire thing as his. It's the perfect example of the series expertly balancing flash with substance.
I could go on and on about them, but I'll stop. It's been said countless times since the DLC dropped, but it really can't be stated enough just how phenomenal these boss fights are from a design perspective. Along with the secret boss (who is currently kicking my ass), these are the best boss fights in the series by a pretty significant margin.