All the captains have some kind of major personality flaw or otherwise. It's hard to pick a captain who has a cool power (which we have seen), has a small flaw, and is interesting enough to be likable.
I like Kenpachi, but let's be honest that he's a bit of a maniac and it's hard to relate to him. He's a loose cannon when it comes to battle, and he has tons of reiatsu but he can't even use shikai (much less, bankai).
Byakuya, I'm sure, is going to be popular, but he's arrogant and prideful, his fighting style involves him standing still in a boring way (reminds me of Gaara, though less crazy). His bankai is just a big version of his shikai, which is kind of boring. All through the beginning part he was extremely dismissive of Ichigo, even though Ichigo consistently outperformed his expectations. It's hard to know where to draw the line between him being arrogant, and him not being smart enough to recognize that Ichigo was something unusual and special. Also, we've seen how heartless he can be when it came to Rukia's execution, and the stupid contradictory promises he's made along the way.
Captain Yamamoto obviously has overwhelming power and an awesome bankai (temperature of the son? Necromancy? You can't block him because his sword will melt yours? You can't hit him because the fire around his body is hot enough to melt your sword before it can touch him?) However, he's a boring old man and he's too arrogant and mistrustful to accept help from Ichigo or Inoue
Shunsui has a sort of boring, lazy personality. To top it off, his shikai is absolutely stupid and hard for even him to control, and if the enemy figures out the rules they can fight toe-to-toe with him no problem. We haven't seen his bankai yet, but we have to suspect it's stupid on a bigger scale.
Hitsugaya is supposedly a genius, but his bankai has been described by several people as being immature and incomplete (and now it's stolen, lol). The one time he used his "guaranteed kill" attack, it was against Teir Harribel and she didn't die from it. So that's a bit of a lie. Maybe it only works against people who are weaker than he is, and he would have been able to kill them anyway. His zanpakuto has been described as the most powerful ice-type, but if you ask me Rukia's zanpakuto is more powerful (and she doesn't even have bankai yet, that we know about). Also, if Hitsugaya is supposed to be so smart, why was he fooled so easily by Aizen and Gin at the beginning?
None of the visords who have become captain again are particularly interesting. Shinji in particular turns out to be a gigantic disappointment because his bankai is pathetic. His bankai was nowhere near as powerful nor as useful as Aizen's.
Mayuri is funny, but he's a creep and his zanpakuto uses poisons, which is a supporting role.
Ukitake seems to have the most normal personality, but we haven't seen his bankai yet and people say he's sick all the time. His shikai only really seems to work against ranged energy attacks, but many enemies don't use such attacks.
Komamura's bankai is a gigantic weak spot. All the real battles we've seen him in, he gets defeated pretty easily (against Kenpachi, against Kaname,), usually because people injure his big stupid bankai.
Urahara is probably my favorite, but his secrecy and personality leave much to be desired. So many problems in the manga would have gone away if he had just told Ichigo and the captains important information ahead of time. Also, we have no idea what his bankai is (his shikai seems useful, generally). We also know that of all the captains he was one of the few who could really stand up to Aizen during his transformations, and that was mostly because of his kido and intense preparations.