just up to you really.. Ive five boxed every instance non heroic so far and most of them heroic with 3 mages a pally and a priest.. I also run warr/pally .. priest/pally .. 3 mages/warlock 2 mages/ 3 hunters grps depending on what i need or how I feel atm. My main grp is the prot pally, priest, and 3 mages tho.

prot pally casue they are best at tanking ae stuff (ie little cc just pull heal kill) also very easy to use dont have to micro manage much

priest due to grp heals

mages due to ease of use (point nuke nuke nuke nuke) and easiest most dependable cc (tho limited to humanoid beast)