I've had some speculation about this subject for a while, and truthfully I just couldn't find a character where playing with him/her won't spoil the other two character stories. I was seriously mistaken at my first choice of order, and now that I've found the right one, I'm going to show you why this order will work best for you.
The reasons why I chose this order is mainly, spoiler reduction, beginners break-in, compared gameplay, and overall, who you like to play with. First of all, is how you want to go about the game without spoiling everything. Of course, even if you go about this the smart way, there will be some spoilers, but thats one of the reason you bought the game in the first place! Still, you don't want to beat a character and find that the other characters wouldn't show or reveal as much as the other, only to be doing a side story at best. (Of course everyone has a significant role in the main story, but unfortunately some have a bigger role than the rest.)
My thoughts on this at first was of course Aqua, Ven, than Terra, this allowed me to play with the character I didn't really feel for first, than play as my favorite character, and finish it up with the character that most connects to the story. This setup was perfect for me since I love to save the best for last and I really thought that Aqua was just like a babysitter that only chased after both Ven and Terra, not really connecting to the main story. I didn't really see any motivation except that this was a mission from her master, and that she cares about them. Her goals weren't really shown at all, but I was horribly mistaken. I'm not going to say too much but it seems she actually has an invisible connection to all of this. Like Kairi, she only hopes for the well being of the hero and potential villain, and she plays a huge but invisible role in all of this. With Aqua, it would be terrible to play as her first as she will possibly reveal everything, plus the interview with Tetsuya Nomura on Kh2.co.uk's website showed that Aqua does have a bigger role, but isn't easily shown until you play as her for yourself.
When you play this game, it's clear that starting off isn't going to be easy, plus you have no idea what's in store or who you're going to face throughout the worlds. When starting off on any game, it's basically common sense to use the most easiest weapon, skill, ability, or attack on the first run, however this is also true for characters. It's clear that I haven't played a lick of this game, so I have no idea who is better between characters, but I can guess who will be more easier to control. The first thing I noticed when watching the gameplay, was that each charter has it's advantage, Terra has power, Ven has speed, and Aqua has magic. Power may seem good at first but with the lack of speed and the possibility of being swarmed by unbirths, escaping and retaliating won't be easy. Magic may be better this time around but it still requires too much distance, so using magic will leave you unprotected unless you can hit all the opponents at once, stunning them. It's kinda hard to do when you're just beginning. The obvious and most logical choice (By no way of favoritism) is of course, Ven. His speed will really help take down opponents and he can easily retaliate against a flurry of foes. He can easily run up and do quick damage as well as retreat quickly for breathing room. It's true that he's lacking in power, but at the beginning, power shouldn't make a difference as long as you can do damage, plus, since the game gets harder the more you level up, Ven should do quite as much damage as normal with out the unbirths overpowering you or Ven overpowering them.
When you compare the three altogether, it's obvious that each has their own talent and weaknesses that are only exploited through battle, so I can't tell you which one is better with out offending some people or being proved wrong once I actually get the game. (Who knows, my favorite character may change during the time I actually play with them) I can only explain each one's weakness than allow you to choose based off of what is least likely to hinder you during play. Terra is obviously a powerhouse hitter, so he'll most likely use heavy thrusts and slashes to attack, this of course slows him down drastically leaving him open for various reasons. He puts his whole body into his attacks, so he slowly turns, or move forward first before releasing the attack, making his attacks a half of a second slower. When you attack, if you get hit, the combo automatically ends, like curaga without a leaf bracer. Since Terra takes so long to load his attacks, he'll get hit before he has time to retaliate. He'll get hit a lot if he's swarmed since he's so slow to attack back, forcing you to retreat before you get caught in a loop. Aqua has the most magical talent of all and uses tricky spells to rapidly and powerfully attack the opponent, but this obviously comes at a price. This dedication to using spells takes away from her overall keyblade CQC (Close Quarters Combat) abilities, and it obviously shows in both how her keyblade is shaped and how she attacks overall. She has a quick three hit combo that is fast, but not as powerful or speedy as Ven's or Terra's. Even in Command style change mode, Spell Weaver, the initial attacks with the keyblade is weak, but magic shot combo finisher is what deals the heaviest damage. I have no idea how this affects battle commands, but hopefully it does the maximum amount of damage that it's supposed to do. Ven has the title of being a quick striker, easily dealing massive combos and rushing the opponent down with a flurry of CQC attacks. With that much speed and combo ability, it's kinda clear that his power isn't that good compared to Terra's. With all that speed, Ven has to use more hits to finish off an easy opponent making the battle more drawn out than it needs to be. The other thing I get is that people are just unfamiliar with his inverted keyblade style, but I won't get into that until I see for myself. Ven also has the tendency to "swirl" with his attacks utterly making them faster, but it also leaves him open at times when he's finishing off a combo or when the enemy is already beaten. His speed also makes him sort of an aerial attacker (Ven does mean Wind in latin) so he is very vulnerable to being knocked out of the air and juggled by bosses or flying unversed. The last thing is that his speed tends to make him rush past his enemies, particularly bosses who jump out of the way, and enemies that are shifty and quick. Still,...even with all of those flaws, comparing the gameplay will be tough since no one is better than the other in actuality, but in your opinion, I'm sure you already have the answer.
The last thing is purely and superficially, your opinion. You can choose from who you like best to who you don't like at all and so on. The pattern is up to you. I shouldn't really have to explain who's your favorite because I know mine and you know yours. With this faq you should have an idea of who you should pick first depending on what you base everything on. If you don't want to spoil the whole game on the first play, save Aqua till the second or third time around, if you want to learn more about it and want to beat it quickly, use Ven in the beginning, and of course the last two are entirely up to you. With six possible combinations you should be able to know who to pick for the last two slots of course. The combination I chose was either Terra, Ven, and Aqua like Tetsuya Nomura wanted it to be, plus they left in exactly that order in the beginning, and my second choice is Ven, Terra, and Aqua. The reason why I chose the second combination was because Ven is my favorite of course, and he's easy to learn for beginners, the downside like Tetsuya Nomura said was that after beating him and going on to Terra, the controls will be a little sticky and slow so I should be prepared for that. This doesn't have to be your combination of course, you can choose anyone you want, but make sure you know what to watch out for when you do decide to go the distance with that combination. After all, it's entirely your opinion, I'm just giving it undeniable reason.
