Warlocks have the best tier sets overall, and priests are second. None of the MoP priest sets were any good, they were all so bland and looked like cheap arena gear, but then they have so many great sets like 5, 6, 8, 11, and especially 13 that it makes up for MoP's boring priest sets. Most classes have pretty good tier sets though, the only ones that usually don't are hunters and rogues, and an honorable mention to monks whose few tier sets are all pretty boring