There are a few characters that are my least favorite, but it seems they're mostly from the newer books – perhaps that's a sign of something. (Either I'm an elitist or the character development is lacking as the books go on…) I'll just note one, though, because it's been a while.
I'm really not a fan of Ashfur. He was fine at first, but the sore argument of “he loved too much!!!” makes me frown a bit. He became possessive over a cat who's somewhat related to him and took it out unfairly on her and her kits. He is full of salt.
ETA: I'm also throwing Crowfeather on here. What better way to say “loyal warrior” than to let an apprentice name himself after a cat he had a crush on who was in another Clan? He treats his new mate badly and his son even worse. He is also full of salt.