Well, if it was a gang thing, Reacher, Joe Pike, Harlan Coben's Win, and pretty much everyone in Jim Thompson's novels--especially the women. Plus Jeremy and Jemima Potts. Those kids were scrappy, and they had the best getaway car ever.
Bruen's Brant, because he would fight dirty, then have them arrested on trumped up charges afterwards. Also Joe Lansdale's Hap and Leonard...but that's mostly because I want to go to the pub with them, Liza Cody's Bucketnut and, as Cornelia has mentioned, Reacher and Win. Plus a female character who shall remain nameless but who is so smug and annoying that I'm hoping SHE would get beaten up first while I escaped through the bathroom window.
Bubba Rogowski from the old Dennis Lehane series. He's got the size, strength, physical skils, and training--not to mention any needed firepower. Oh, and he'd be the most fun to talk to in the post-brawl aftermath.
Spiderman. The guy could just wrap everyone up in a web until they got sober, and that way no one would get hurt. And if you say, "Well, duh, that belies the whole point of a barroom brawl," I say, "Well, duh, I'm a girl and way too dainty for such things."
But that didn't stop me from putting one heck of a barroom brawl in something I wrote recently. Wild Bill, Buffalo Bill and Calamity Jane were all there. Historians say Wild Bill wore his guns backwards and drew cross-handed, which is why, at least from the standpoint of physics, he was so much faster on the draw than anyone else in his time. You can see his pearl-handled pistols sticking out of his belt in the photo below.
Come to think of it, maybe I'll just have Wild Bill on my team, but darn it, he's not fictional.