I would have to say the first tier in wrath: http://www.wowhead.com/itemset=787#. It looked amazing and i think the set bonus was rly a nice buff for warrior tanks. Skulls on the shoulders and getting the shield from pathwerk or gluth: Hero's Surrender Looked to fit this set correctly. Remember i tried for weeks to get all the parts, but never got any. Then one week every token dropped and i won the roll for them =)... Good times..
My Favorite Tier of Gear was the Shaman Tier 10 cause I liked the spirits the popped out of the shoulders. It was really cool when you were under the effects of a transformation item to see the Shoveltusk spirit appear.
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Avatar Raiment (Tier 5 Priest set), not only due to the badassness of it, being an angel and all judging everyone with my equally badass Staff of Immaculate Recovery. It also gives you wings. I just extremely liked the art and halo effect a lot.
My favorite tear set is Nightslayer armor. ( Nightslayer Armor ) I always loved the helmet, but I also like that there is nothing glowing or sparkly on it.
Paladin's Judgement Armor - Tier 2 Raid Set
Tier 6 Warrior was the best, mainly because i loved how beastly it looke.
I am a warlock. The only set of armor that really made me stand out and made my enemies cower in fear was the Tier 6 Malefic Raiment set. You can't beat standing around, having people admiring your armor when suddenly you sprout demon wings. It is just beautiful. This was the most unique set of armor I have ever owned.
Priest Tier 5. It's only mildly rivalled by the tier 6 non-set-recolor in terms of looking "holy". I cringe at the sight of Shadow Priests using it.
The paladin Tier2 is freakin epic!The designs nowadays with the ugly color schemes? WHY Blizzard.. Go back to the glory days with T2!