The data battles were all very fun and challenging. The real challenge is getting through the Cavern of Remembrance though...
Here's my experience from playing the data battles from FM:
Xemnas' battle is long, but straightforward, it's a fight against him at the Skyscraper and his final form back to back.
Xigbar wasn't too bad, it was pretty much the same fight as the one in TWTNW save for a few new moves.
Xaldin was pretty hard, but I got the timing on the blocks down. Didn't change much from Beast's Castle.
Vexen was an utter pain, you're fighting him, Anti-Sora, and his shield at the same time, beating him feels pretty rewarding.
Lexaeus wasn't that bad, well timed blocks are key because one hit will leave you with Second Chance.
Zexion's fight felt really hard for me because FM was in Japanese and I just did guess work for which one the dispel command was. Also random-ass meteors.
Saïx was pretty straightforward, didn't change much from TWTNW.
Axel was pretty annoying, for the love of god disable Quick Move. The battle was pretty much all twitch reflexes though, gotta make the decision between Dodge Roll or blocking in a split second.
Demyx was just Wisdom Form Firaga spam for the entire battle, I couldn't think of any other way you can kill clones fast enough.
Luxord can be pretty tricky as the last roulette he does is actually fairly hard to land, other than that he was pretty easy.
Marluxia's Absence battle was hard, but his data battle was stupidly easy. His attacks don't damage you directly, if you're level 99, he'd have to hit you 99 times with some of the most easily blocked attacks in the game to kill you. Oh the counter is also able to be replenished, pretty hard to lose this one.
Larxene is probably the hardest data battle for me, her attacks are so fast and flashy that it's really hard to keep up with what she's doing.
Roxas was one of the harder ones, he counters every finisher you make so can't go all-out, one stray light beam could make the difference of the fight. Also his dual raid move is pretty much unblockable, sometimes he just never uses it though, other times he'll start using it as a counter attack and it snowballs from there.
Looking forward to playing these fights again in 2.5!