Side scrolling beat'em ups are a genre I don't know really know too well. I played Golden Axe and Streets of Rage on the Megadrive when I was a kid but never really went beyond that.
What games are generally considered to be the best in the genre? I'm in the mood to play something.
Final Fight
Streets of Rage
Dungeons & Dragons: Shadow over Mystara
Dragon's Crown
I'm not that big into the whole side scrolling brawler genre, but those are the big ones I usually hear mentioned. I also had fun with King of Dragons back in the day due to the whole leveling mechanic, but I admit that the D&D one and Dragon's Crown handled it better.
The best part of them are the multiplayer aspects, and outside Dragon's Crown or a few indie Steam titles, you don't really get online multiplayer with them.