There's a saying that "you can tell a man by the company he keeps." But not in this case. Your fiancé is kind, honest and funny—an all-around great guy. His best man, in your opinion, is not. You've tried to like him but his immature, self-centered, frat-boy ways drive you nuts—the guy has a rep for getting wasted at weddings and launching into bad-but-not-funny Beyoncé dance moves, and he likes to hit on pretty female guests, whether they're single or not. Since you can't get rid of him, we'll tell you how to cope instead.
Don't try to demote him.
He and your fiancé go way back, so unfortunately there'll be no changing of the guard—he's the male lead of the wedding party. Just as your guy has no input into the status of your maid of honor, you have no input into voting off his best man. Don't get into a fight about this with him—it'll only make things worse and won't solve the problem.
Speak to your fiancé.
At this point in your relationship, it shouldn't come as a surprise to him that you don't like the best man, who's obviously close to him, often a brother, childhood buddy, or college friend. But rather than rattle off a list of his offensive personality traits (too many!), narrow down what's bothering you about having him in the wedding party. Maybe it's the toast—that he'll drop F-bombs, tell inappropriate anecdotes about your new husband's pot-smoking past, mention his exes, etc. Ask your fiancé to tell his friend to keep it clean and remember that Nana's in the house.
Consider having a heart-to-heart with the best man.
Maybe the solution is to have a chat with him yourself. Keep it simple, polite, and, again, specific about whatever you're worried about. Understanding how much this day means to you (as if he wouldn't know otherwise!) may encourage him to be on his best behavior.
Sit down with both of them.
Keep calm, be mature, and discuss what's bothering you. Ahead of time, make sure your fiancé knows he should have your back. It shouldn't end up as you pitted against the two of them. Let your guy do some of the talking too.
Call a truce.
It's not unheard of for a best man to also loathe the bride. (Yeah, you.) Maybe he's annoyed that you've taken up a big chunk of his BFF's (your groom) time since you started dating. Or your personalities may be like oil and water. So if the feelings are mutual, agree to be civil toward each other at the wedding. Except for during the ceremony and picture-taking, you don't have to be near each other all night! When it comes to the toast, he'll hopefully make it more about the groom ("Here's to the greatest guy in the world being as happy for the rest of his life as he is today") than about the two of you. Can you imagine what he'd say?