*bangs head on desk* I was hoping this was going to be a simple q&a, but it's now turned into another opinion bashing thread, for what I think.
It all varies. People here are
highly opinionated on here.
My two cents:
Terra isn't
terrible, but seriously not that great. At least in my opinion. Why? We've already
seen a character do bad things, fall to darkness blah blah blah, what have you (I refuse to go into this anymore). But it was executed
better with him (I think so), and with Terra it was predictable right from the get go. No, it was
not identical with the two, but Terra's was dull in the sense, I already
knew, for the most part, what would happen.
Ven was my favorite of the trio. His personality and story reminded me a bit of Sora's, but he was also unique. At first he was trying to find Terra. Then crap happens and after RG, just wants friendship, feeling hurt because his two best friends don't seem to believe in him...or something. Interpret that as you wish.
The fact that he's also seriously important to Sora's overall story is also a plus- who actually knew Sora would be sheltering his heart? Don't forget the whole Vanitas thing.

Evil Sora Clone wheee!!
Aqua was dull to me up until the end. Yeah she could fight. But she had a slightly "What I/the Master say(s)/do(es) is right. No exceptions" attitude/personality, which bothered me. (I don't like people who act like that. It's obnoxious.) She redeemed herself by showing her true colors at the end of her scenario, as well as the last episode. She actually
had a unique character to her over the typical female role seen in the game, as stated. Her ending as well was entirely unpredictable.
Just my two cents. Terra was predictable all the way through. That's a key issue. So is the fact we've already seen something like this executed before. He also lacks (I personally think so) some defining personality trait. He's flat, boring, etc. Just a "fallen hero" without something that makes him special.
Other than the fact Xehanort completely takes his body. Yikes. But I knew he was connected in some way to the current Xehanort we love to hate.
He wasn't terrible. But he's certainly not the best. Though...he outranked Eraqus by a long shot for me. >_>; Ugh I couldn't stand him. But that's an entirely different subject, now isn't it?