Mine has to be the warrior t6, ever since I first saw it back in tbc I've REALLY REALLY REALLY wanted it.Ofc I never got the chance to raid anything past kara then, but as of late with my guild trying to get me the warglaives, I took the opportunity to start collecting the pieces. Got 4 of them now, only another 4 to go untill it's complete.Anyways, the reasons for the onslaught set being my favorite one is because wearing it, you truly look like a tank. Especially if you got the Bulwark of azzinoth aswell. You really look like a fortified slab of meat wanting to get punched just so you can laugh back knowing your armor is impenetrable.
I didn't start really playing til the end of LK, but I would still have to go with t11 or t12 for the Death Knight. The look and color scheme of them really bring out the, "I'm going to kill you and eat your soul" part of them. That's just me though.
My favorite set was the Shadowblade set (read: gimp suit) for rogue. It was really, really thematic and fitting as far as I was concerned. I'm seriously wondering just getting that set for my rogue with the justice points. Never quite got to have it!
The tier 2 sets were among the best IMHO. Especially the Judgment set for Paladins and Bloodfang for the Rogues.
Paladin T2 because it goes so well with the ashbringer.
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My favorite was Absolution Regalia(Priest tier 6) I loved the hood the most, made me look like a person hiding in the shadows, NEVER got the helm but ALWAYS wanted it :(, the heads on the shoulders and the robe look pretty cool looking also :).
Paladin Tier 5 Raid set."I'm blue da ba dee da ba die, da ba dee da ba die..."
For me it was always the T2 Bloodfang Armor. The Red/Black color palette and sharp-edged designs acted like what those bright colors on poisonous creatures in the world had to warn enemies. "Don't mess with me. I will kill you." The black smoke coming off the shoulders and head was subtle, adding to the set without demanding attention.
T8 Rogue set. the Helm was terrifying and the shoulder animation was amazing
Pally T1... cause who doesn't like running around looking like a banana? Or like you are going to a disco? Come on! Its awesome!
Judgment Armor This is indeed a great setAnd this one The Ten StormsDungeon set http://www.wowhead.com/itemset=186#.And a uncommon set i enjoy as bonus :) Fel Iron Plate
Warlock S3 (aka T6). The wings that shot out while wearing the tier helm were awesome! http://k.min.us/jneqZ8.png
One of my favorite armor sets has got to be the Tier 6 Paladin armor, the shoulders and the helm just look freaking awesome and the rest of the set just serves to complement them.
Plagueheart Raiment, the T3 warlock set: Plagueheart Raiment So badass.
My favorite set is the Judgement Armor (Tier 2 Paladin Set) : Judgment Armor
Paladin tier 6 because it looks plain awesome; gold and glowy!