Quite a few, I see...
1. My least favorite character of the gang of five (or seven) is Cera. Her prideful, sometimes quarrelsome attitude often gets to me, despite the fact that it comes in handy at times. Perhaps her father, Topsy, who is my least favorite character overall, could cause such an influence on her. Mr. Threehorn/Topps is a character that I quite dislike for obvious reasons, though of course for other reasons I don't hate him nor Cera, or any of the other characters for that matter, as I don't feel it right to hate a character in any media unless you absolutely have to.
2. Probably Chomper in movie V, when he is singing "Friends for Dinner". The kid playing him there clearly couldn't act, let alone sing, for goodness sakes! Other than that, I don't think any of the characters' singing voices (not even Petrie's) are that bad.
3. I might have to re-watch them again to answer that question, as it has been a while since I've last re-watch them from beginning to end, but for now I'm probably going to have to go with 13. For me, it's not because of the darn yellowbellies (which I didn't mind to an extent), but more because of the simplicity of the film than anything else. Going back to what Ducky123 said recently, while it had a good message and all, it could've been done better than it did.