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My personal favorite, obviously influenced by the class I played at the time, was the Priest Tier 5 set, the Avatar Raiment, released during the Burning Crusade expansion. Between the auric, argent, and indigo color scheme, the waist chain, the prominent hood and the wings on the shoulders, this set really made a Priest stand out. It matched the feel and flavor of the class, and for once gave priests a cool looking set. Especially in contrast with the Vanilla dungeon and raid sets, this set showed the design team taking a step away from the previous design philosophy and going for something truly new and unique. I'm ordinarily not really that much into the appearance of my character, but even for me at the point in the game in which this was released, it really struck a chord with the players and gave us something to be excited about when looking at their characters.
By far, my favorite gear has been the Lightbringer set, especially when combined with this mace. It has such a vengeance feel to it, and was the first set that made retribution feel truly just. Also, the paladin tier 10 specifically the ten man version has a nice feel to it when combined with Neverending Winter and Professor Putricide's last word. My view point may be a little skewed since it was what Highlord Fordring was wearing and I find him a bad ass however, but these remain the most awesome looking sets so far.
I'm working on the latest set for pvp so I can own all noobs I see.
I enjoyed my DK Tier 10 set the most, just seemed very fitting, however the helm had to be turned off, which is usually the case for me.
Paladin T2 Best looking set in game Judgment Armor
since I start wow after Wotlk, I like the druid T11tier the best, just because it got wings and feather dropping effect
My overall favorite is the T10 for Rogues. It just made me feel awesome whenever I wore it with my daggers from ToGC. I am starting to fall in love with the T11 for warriors though cuz it looks amazing!
I really like the look of the new Tier 11 set for Paladins. It makes them look like one of the Rock Giant's from Hellfire Peninsula. I really like how the shoulders came out, since IMO the shoulders either make, or break, and armor set. Very well thought out and well done Blizzard.
Druid tier 10, just because on tauren... you had massive flesh eating plants on your shoulder that could eat a gnome whole. lolthe helm wasn't the best, Mage tier 11 looks like it should be in some sort of horror film cause its so cool. but tops it is the lighening wings from druid tier 11 on Tauren.....they engulf the whole Tauren. but that is just my opinion, everyone has their own
Rogue T5, Deathmantle set was the best! Mask of the Phantasm anyone?
I love the first raiding tier in wotlk. It looked very good, and it really felt unique to go inside Naxx with it.
Vestments of Absolution, Tier 6 Priest set was my favorite. Vestments of Absolution . The work on the shoulders is simply stunning, particularly for a shadow priest, and the hood fit really well into the theme.
I know some may think its lame, but I have to go with Shaman T10, the Frostwitch in my mind was the most Shamany looking of the Shaman Tiers, which is why I still wear it from time to time where my Guild can't see ;-)