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My best is Lawbringer Armor Tier 1 Raid set for pally... It makes paladin worth a name. It's just the right size and it makes paladin what they realy are, empowered by the light, all shining and reinforced looking. Very strong looking set.
I liked t10 because that was the tier that was out when I finally got to 80, before Cata raised the cap.
The priest dungeon set 2 (tier 0.5) is my favourite (or you can tell that almost every class really). It took almost ages to get it done, but was worth it. Quest line is epic :) Getting whole set is long time and much effort. Look is awesome and reminds me about old days when I started playing :) I still keep whole set in my bank.
Has to be Netherwind Regalia for me - Really, all of the Tier 2 sets, but purely Netherwind because my mage always has and always will be my main... But the patch day for 1.9 when all of those armor pieces got a much needed graphical overhaul (along with other cool epics such as Staff of Dominance and Perdition's Blade) I was blown away by the new shinies I was seeing everywhere. It was the moment Blizz really started to step up their game with making epic items LOOK epic! Also the first time Netherwind Focus procced for me I could barely contain my joy :)Edited for typo
My favourite Tier Set is by far Priests' Tier 6. Why? Because it's freaking hot. And cool. At the same time! It gives goosebumps while you melt people's faces off!And worn by trolls it breaks the limits of awesomeness.
My favorite Lock´s Tier 5, http://www.wowhead.com/itemset=670#. of the Malafic. Why? I have a respect to Undead with this set and like when player moving and wings show out.
Paladin T12 because I love dress !
In my opinion the best set in world of warcraft is Bloodfang Armor, he truly reveals the essence of the Rogue, so-much more enjoyable moments was associated with the time and effort put order to get it =)
Tier 11 for Rogues is my favorite, simply because of the awesome turban. Get yourself a flying carpet and you have the makings of a Disney animated movie!
T6 Paladin ftw, nothing looks better than a pally with wings poped in that set
Heroes' Redemption (10-man paladin T7), because the color scheme was perfect for my blood elf paladin and it really gave a feeling of wearing heavy plate and being a righteous crusader.
Definitely the Rogue's Deathmantle set. It was just uber epic! The helm looks just like a rogue's helm should look!