Because I'm a derp (thanks to Days). Just some notes on which characters are best at what, fighting styles, etc.
Baseline for comparison is mission 07, Twilight Town, collecting hearts. It's simple, quick at this level, and has a nice array of enemies.
Equipped: Ultimate Gear+; focus on strength and level panels, and the basic abilities + a couple of addons. I'm a button smashing combo person, not a magic caster. Especially since my L button is fucked up, so my magic shortcuts are as well. ALSO, because some people have asked, I prefer Pandora's Gear+ over Ultimate, but whatever.
Xemnas - Good once you get over the fact that he hovers, rather than walks. Also the fact that every time you hit X, you hear "HWAH *lightsaber hum*". ANYWAY. Excellent for groups of Heartless, though he tends to get way up in the air with his combos, if there's enough stuff to hit. But he destroyed both main groups of Heartless really quickly, with lots of munny and mission points and items cascading down. Also, I kept forgetting to use glide with him, because hovering. :|
Xigbar - Xigbar is like... a unicycle. Some people will prefer him, and get the hang of it right away, and people like me will fall off all the time and bash into things. Between the shit he does while firing regularly in random directions and sometimes hitting Heartless, and the reload time, it took me like five times longer to take out the two main groups than it did with anyone else, pretty much. I actually had auto-life kick in, because I was so busy trying to actually hit something that I didn't notice my health. And his limit is in only one direction, and sometimes you even get it lined up with something to hit! :|b
Xaldin - For all his personality failings in the game, he's pretty kickass in a fight. His weapon gives him fantastic range; I was actually batting attacks away before they could hit me, and took out the first big group of Heartless without taking any damage at all, even from the Possessors. Might be slightly faster than Xemnas.
Vexen - Moderately slow attack speed. Runs like an old woman, or like he's got a sizable dildo up his ass. Awesome limit, though his voice for whatever it is he says to announce it sounds like they got Quinton to do it, but kicked him in the balls first. BUT. Good at smashing things, and seems to be better than some at dodging magic, even while standing still. IDK I LIKE USING HIM BECAUSE I LIKE VEXEN, but some people might not find him awesome enough in combat.
Lexaeus - With Ultimate Gear+, his weapon is huge and ridiculous, but that's neither here nor there. Also he runs strangely and sounds like he's giving birth whenever he attacks. High strength, low speed, blah blah blah. Excellent for smashing things, but only one or two at a time.
Zexion - Not made for physical attacks, but for someone with a book for a weapon, does surprisingly well. His limit is awesome, and basically rains death down on everything around him.
Saix - Hard hitter, but has a difficult time staying in the same area he started in. His limit is similar, but with even less control, as he mostly bounces all over the place causing lots of damage to anything that happens to get in the way. I wish I had him for the Battle of 1000 Heartless.
Axel - Less of a hard hitter, but his speed makes up for it, and he's got the best agility so far (unless that's just me discovering that yes, I CAN make everyone turn on a dime while hitting things). Also he sets things on fire when he hits them sometimes. (The other people with obvious elements do this as well, but fire.)
Demyx - Surprisingly good, considering he's more of a magic-user. He uses a combination of sitar-whacking and sound waves, for physical combat, and can take out multiple things at once that way. I wouldn't want him on his own for like, a boss fight, but he's pretty good. Also, he runs really strangely with his sitar, and glides even more strangely.
Luxord - HOLY BEEZUS THIS MAN FINALLY HAS A USE TO ME. He took out the first trio of Shadows in this mission in one hit. And while his HP got low enough in the first big group of Heartless for me to use his limit, the limit itself more than made up for it; he targets individuals or pairs, and then like. Explodes them, if they're within range (which isn't too hard to manage). He's AWESOME, though he's not as direct as some other people, because he can't turn or move once you start a combo, really.
Marluxia - He's fast, he's got incredible range AND strength, and he's fabulous. He's so fabulous that I have no idea what his limit is because his health didn't get low enough and he kept dodging automatically. Mmmm, delicious scythe.
Larxene - SOFAST. Her speed is shown at max, same as Xigbar, but she's... not a unicycle. She's just kickass. And a bitch. She has longer combos than everyone else, it seems like, and hits hard and fast, and her LIMIT. Ohgod her limit. She just pounces on things and kills them for a few seconds, and then dashes around with lightning and awesome and death and yes.
Roxas - All around average. Familiarity with his fighting style from Story Mode will probably make this a comfy choice for a lot of people, and the pacing of his combos with certain weapons will also help, since they'll be familiar. The averageness of him is handy, though, because you can swing him more towards strength or magic depending on weapons and levels and such (as in Story Mode, obviously), which will put him on par with the other members for strength or magic.
Xion - See also: Roxas. No, really, same everything, except hair and voice.
Riku - Very similar to Larxene in speed and number of strikes per combo. His weapon doesn't change with different gears. :C
Goofy - Similar to Vexen, but with much higher defense, and minimal magic skill. His blocking action is silly, but he hits hard once he gets going.
Donald - So worth it, just for his limit (the ____ Flare, I think it was, in KH2) and his ducky run and his GLIDE (with flapping!) and the fact that everything he's fighting is the same size as him, or larger. And I didn't even use magic with him.
FINAL VERDICT - Best for button-mashing: Xaldin, Axel, Marluxia, Larxene, and Riku. Best for general awesome or neat tricks: Zexion, Demyx, Luxord, and Donald. Best for magic: idk I didn't test that, but I might later. I'd just go with the stats shown and go from there.