I thought of mentioning TMNT and Double Dragon, but I'm not sure how influential the former was. It's been a while since I played it, so I can't say if its on par with Capcoms late-genre efforts like Punisher/Armored Warriors/AVP, and at least I don't think it moved the genre forward like some of Capcoms games did. I think I'd relegate it to "honorable mentions" along with Silk Road, Sonic Blast Man 2, Ninja Base Ball Batman, Captain Commando, etc. There's also Battletoads, but I'm not sure I'd count the Nes game as a beat em up. The SNES one is a beat em up for sure, albeit an awful one. The arcade version of the original is actually a pretty solid game, but like TMNT its not the most innovative thing out there (aside from the unique sense of humor).