One of the most frustrating things a groom like you might have to deal with in your life is a best friend or good buddy that doesn't like your girlfriend. That's understandable, of course - girls come and go, but you'll always have your friends, right? That's all well and good right up until the time you decide to get married to the woman that one or more of your buddies just never quite clicked with, or maybe even openly dislikes. This puts you in the awkward position of refereeing between them, or even constantly defending one to the other.
Frankly, though, your buddy is just going to have to deal. This is one time when it's not a competition; he can't win this one, because you're not going to marry him. Even if he's reluctant to say exactly why he dislikes your future wife, that's information you need to get out of him. The reason could be anything from a remark she made that he didn't like, to an argument they had, the way he thinks she treats you, or the simple fear that he won't see you as much after you get married.
Once you've isolated the problem, talk to him about it. You don't have to have a big dramatic sit-down chat or anything, but next time you're out having a beer, buy a round and bring it up. Ask what he needs in order to help this wedding go smoothly; bribe with groomsmen gifts if you have to. Maybe she can get into his good graces by introducing him to one of her pretty bridesmaids, or just having a conversation with him herself - after all, there's nothing quite as reassuring as her telling him straight up that you'll still get your poker nights and you can still come over to watch the game. Sure, we don't like to admit that we're insecure about stuff like that, but sometimes a guy just needs to know his buddies will still be around, and it's an understandable fear that marriage might change that.
You should have a chat with the future wife, too - don't ask her to change (just don't), but if there's something in particular that your wife does that offends your groomsman, let her know. Use your best judgment on this one, of course. If she makes catty remarks about your buddy's clothes or girlfriend, it's within reason to ask her to cool it down when he's around so as not to be rude. If it's something like that your groomsman can't stand her annoying laugh, though, keep it to yourself. Your goal is make peace, not encourage more fighting.
If you've got the time and the willingness to tackle the potentially sitcom-y situation of making them bond with each other over a shared interest, it wouldn't hurt to try. If they just spend some time hanging out, maybe they'll find out that they really can like each other! Hey, it works in the movies. Just make sure it doesn't work too well - poker nights with your best buddy and the Mrs. might get a little awkward.