I have to go with the Rogues T7, Heroes Bonescythe set as my favorite. I really like how it looks. Although their T2 Bloodfang set is also very good looking.
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Tier six. It was, in my opinion, the last major tier of set design that was highly unique for every class without any of the sets being egregiously over-the-top. Paladins looked like holy crusaders, warriors looked like actual tanks, the entire tier just had really solid desigsn backing it.
Warlock Tier 6 is by far the best in my opinion, The wings on the back make it look so epic I only wish there was a way to trigger them to come out xD and the helm with the horns is quite nice as well really making the demonic feel of a warlock come out.
I like the Warlock T5 Set not only because it was just awesome looking but it was the first set I got.
Priest 70 pvp Brutal Gear.
My personal favorite is the t10 gear for warriors just because I loved the way the heroic gear looked like on my main.
Valorous Scourgestalker Battlegear(T8) was my favorite because it was firstly my fav color red xD and it had that deadly hardcore hunter theme skin and Ulduar was the best raid ever. :pPS: honestly I liked the fact that Blizzard kept the 'stalker' suffix on all my hunter tier sets, but unfortunately Tirion said no more stalking. :(
My favourite set is the Sanctified Bloodmage's Regalia!Not only was it the first full (well, 4/5) set I completed on my mage, but it also looked absolutely incredible on my Undead - without looking too flashy or over the top (i.e. no glowing helmets... sure the shoulders glowed slightly, but it was only slightly). Also I LOVED the set bonuses: I had an incredible amount of crit back then... over 1500 crit rating before the change to molten armour as I had a lot of spirit as well as actual crit rating so the 12% haste buff was always up!
I've always been partial to the look of the Tier four, particularly on druids.
I like the T2 Paladin set. looks awesome and looks like what a real paladin should look like with colors like the paladin charger. matches really well.
Paly Tier 2 hands down. Judgment Armor
Shaman t6 was my favorite, Skyshatter Regalia. Probably my favorite shoulder graphic for a shaman tier so far, perhaps t12 will upstage it when they finally release the shaman models.