Streets of Rage 2 is my joint 2nd favourite game of ALL TIME only narrowly missing out to the sublime Shenmue.
What upsets me as a 2D side scrolling beat em up fan is that we get avarage games like Turtles in time and Castle Crashers on XBLA, but theres no Sign of a Streets of Rage 4 or Guardian Heroes 2 anywhere.
Castle Crashers has shown that theres still an appetite for classic side scrolling beat em up's.
If a game where all the characters have the same moves set and the enemies are bland (ie Castle Crashers) can do well then imagine a 2D Streets of Rage 4 on XBLA/PSN, Streets of Rage's characters all had a unique moves set.
Picture Streets of Rage 4 in your head with 4 player co-op and the cast was........... Axel, Blaze, Max & Shiva (yes, Shiva see's the errors of his ways and joins the good guys) it would be MINDBLOWING.
Heres Axel vs Shiva from back in the day (the fight and the music are fantastic) http://www.youtube.com/watch?v=aELhHGUEhz0&feature=related
Oh, i voted for classic 2D and my joint 2nd fave game alongside SOR2 is of course Guardian Heroes (Sega Saturn)