I beat my game a while ago, but here are my thoughts:
By far, my favorite to use was Aqua. Although her physical damage output was low, I could easily find myself dealing damage fast using magic. Sadly, having only 8 slots to put different combos into is not too great. It's like having moveslot syndrome. Otherwise, combining the right commands to get good abilities wasn't too tough.
In terms of bosses, I ended up beating the Vanitas Sentiment only with Terra however. He was really tough, and the easiest way I found to do so was just spam Strike Raids while using Vanish and sitting behind one of the large rock structures.
I tried Mysterious Figure afterwards and I have to say that he was by far, the toughest boss I've ever faced. I don't think I would be able to beat him unless I sat around and kept retrying it until I would win, especially since I'm running Terra.
I do enjoy Terra for his large damage output and bulk, but the fact that he can't Dodge Roll or Cartwheel quickly makes him inferior pick when fighting bosses.
Ventus is great because of Dodge Roll. His damage output is the most versatile, hitting both sides of the spectrum with equal damage. However his strongest trait is his natural attack speed. I like that since that means I can string longer and faster combos. However, that also means that it even his finisher isn't the strongest thing in the world.
Out of all the commands, I love Vanish the most, since it does not require leveling up and since it basically makes you un-lock-on-able. Otherwise, the other commands that take up two slots are fun only for the animation.
I especially like it when you fight the Trinity Armor and the camera closes in on Ventus because he appears to be bouncing to the beat of the music. Plus his battle stance isn't boring like Aqua's (he spins his keyblade around, fuck yeah). Jokes aside, this game is totally worth buying. It has the longest play time in the entire KH series, totalling over 50 hours of play time (and I haven't even unlocked everything yet, and I've still more to go). Buy this if you plan on getting yourself a very good Real-time RPG for a handheld like the PSP.