Well, in order...
Robb Stark I consider to be an utter chode of a character. I really get the impression that he was just supposed to be a replacement for Ned, but the sheer fact that he barely features at all in Clash of Kings kind of reveals what a nothing character that he is. Even his parts in series 2 are pretty token and I really feel nothing toward him, which makes him worse than Joffrey in my eyes... instead of being merely hated, he is instead boring, in a way that Ned totally wasn't.
I actually kind of liked Renly Baratheon, even if his idea for a 'Rainbow Guard' was a bit poncy and gay. But then again, so is he, so I guess that's just good characterisation. But at the end of the day, the point of him is that even though people like him, he's just too flowery and faffs about too much to really be an effective king.
Stannis.... I really have mixed feelings toward Stannis, but he is definitely more interesting than Robb. He's definitely a character that you can both dislike and pity at the same time. He's more of a tragic hero than the others, his hatred and mistrust of people causing him to be lonely and without an heir, causing him to turn to a seductive woman stringing him along hopelessly. Although he's by far the best candidate for king in my eyes (despite his ever-weakening mind), that says a lot more about the other kings than him.
Balon I know very little about really (I've only encountered the first two installments of the series and the book, so I'm guessing he becomes more fleshed out later), although he has a savage kind of simplicity to his being-a-dick that I find quite appealing.
Joffrey... what can I say about Joffrey. He is an absolute ****, but in a way that makes him the highlight of every episode I watch. He's a wonderful character on every level, one that people love to hate- his acts of cruelty are both fascinating and really get you involved in your absolute hatred of him, which makes the moments when he is revealed as a sniveling child all the more gratifying. Definitely one of my favourites, after Tyrion.
Daenerys, however,has definitely endeared to me the most, going from being a simple and submissive bargaining chip by her brother and transforming into a brutal woman that hints her future as a ruthless queen. Personally, I love Daenerys, so I'm totally rooting for her to win in the end.
TL;DR
Robb's a boring arse.
Renly's ineffective, no matter how much I like him.
Stannis' status as a tragic hero endear him to me.
Balon I know next to nothing about.
Joffrey's a ****, but one that is absolutely delightful to watch in action.
I'm rooting for Daenerys.