Well, when it comes to side scrolling beat em up's i think that Streets of Rage 2 is the best of all time.
Better than Turtles in Time, Final Fight, Double Dragon, Golden Axe ect.
And as for castle crashers? well, it has sold a lot and has 4 player, but the bottom line is that all the characters moves are the same but in streets of rage 2 you have 4 characters all with unique move sets.
3 Games rape castle crashers IMO (Streets of Rage 2 & 3 and the greatest side scrolling beat em up/RPG of all time......... Guardian Heroes)
I find it baffling that there are so few people in the VGchartz league that own Streets of Rage 2 (it's only 400 MS points and has online co-op)
And as for music.......... i still rate Streets of Rage 2's soundtrack as the best of the 16 bit era.
Yuzo Koshiro is a music legend.
@ Graham, yup, kid chameleon is there (one of the most underrated platformers of all time) and vectorman 1 & 2 are there aswell.