My personal favorite item set is the Hunter arena season 9 set, Vicious Gladiator's Pursuit. The helm itself looks really cool - basically a flaming samurai helm with a pirate eyepatch. Very nice for those undead hunters that want to hide their non-existent lower jaw.
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Priest T8. Nothing says "You know you need me" quite like it.
Tier 10 DK, because look at it... it's such a badass set.
I would have to chose the rogue tier 10: Shadowblade's Battlegear Sanctified Shadowblade's Battlegear. It looks awesome on heroic version, it resembles poison (wich is a rogues best friend :D). Has nice bonuses, and even thow it wasnt all that easy to get it also wasn't difficult.
My favorite tier of armor was Warrior tier 3. The rehashed versions in WoTLK NNaxx didn't have the colours right
warrior tier 2 - an intimidating looking set with the best helm
Priest T6 (http://www.wowhead.com/itemset=675#). Clear and simple set but kind of grand and a touch of darkness. I loved that set!
I really like Warrior T6, cause that pretty much screams "I'm a 'effing tank and you dont wanna mess with me" Especially combined with the shield from Illidan..
Priest t5 has yet to be bested. The head and shoulder slots look amazing, not the homogenized tier sets today.