I guess I have to complicate things. It depends on how much work you want to put in a game.
If you actually want to play a fighting game, and work on it, and not be sure how combos are gonna work, Super Street Fighter IV is your choice. Have fun learning those combos though.. Other games like this are Blazblue, or if you're really dying to be hardcore, Virtua Fighter 5.
If you want to not put a lot of work in, May I interest you in Dead Or Alive 4? Easy mechanics, simple controls, moves have more flair.
Basically, all I'm saying is, if you just wanna mash some buttons, then the easy choice by far is Dead or Alive 4, because at a casual level, Street Fighter, BlazBlue, and Virtua Fighter 5 will be frustrating. Not fun learning a bunch of fighting moves when a button masher will achieve the desired effect, and has a much greater casual appeal.
Oh, and Dead or Alive 4 is probably multitudes cheaper.