We certaintly have to give credit to the godfather of all "one on one" fighting games. Which is ofcourse Street Fighter II. Since when in gaming history have they announced 10 (kinda exaggerating, but not by far!) different variations of the same game!
Now if want to talk about sidescroll beat'em ups then Double Dragon obviously takes the cake in that category since it was the original. These were the 2 games that started it all. All the others simply followed their footsteps and added their own unique twists. Which is what marketing is all about if you want your business to stay alive =)