Out of those, I would say MM spec Hunter is your best bet in 5s. Insta-gib someone from stealth, then try to win the 4v5. Enhancement shammies lost a significant amount of burst this expansion, although their damage isn't exactly bad as far as I can tell.

As far as ladders go (atleast on EU), 5s is quite bad since there are simply no casual players. All I've ever qued in to is teams with viable 2-healer comps and full gear, with above 1800 MMR. It would appear that the only reason anyone plays the ladder anymore is to push for the 2.2k achievement required for Arena Master. 3s is a bit better, and 2s is by far the easiest to get wins in. Or atleast it was when trying to cap conquest.