Favourite Beat 'Em Up

My personal favourite is Capcom's Rival Schools series.

Rival Schools: United by Fate was released on the original PlayStation back in 1998 and as the title suggests revolved around high school rivalry. What I loved about the series is that it actually had a proper continuing story in-between fights, something that was quite rare for a fighting game back then. This gave the game huge replay value, as every character story was different and to unveil the full picture you needed to complete every school's narrative. 

The character design was awesome and had such unique fighters that i'm surprised it doesn't have a bigger fan base.

It spawned an Udon comic series and one sequel, Project Justice, for the Sega Dreamcast which is my favourite of the two games.  Sadly that was the end for Rival Schools, although various characters have popped up in the Capcom Vs series and also Street Fighter as DLC.