I've been enjoying Mighty Wardens, but I only play local with my roommate. Don't really know how good the online community is. Fun beat-em-up style RPG. Just straight fighting the whole time though.
I also recommend street fighter volt HIGHLY competitive fun game that uses Game Center multi. Bunch of people play it too. Very fun.
I had a lot of fun with Air Wings. Other games, not really shooter/action type though, but turn-based: Dash Race Soctics League Online Lead Wars