My favorite ones are:
Capcom vs SNK 2
Guilty Gear XX etc
Mortal Kombat 9
Marvel vs capcom 2
I havent bought Marvel vs capcom 3 yet but i will probably get the Vita version when they have a sale.
I love fighting games, but i dont play them online cause it require you to study the game and the characters and while i love to watch i cant be arsed with the practice necessary to pull off all the crazy long combos.