The Dreadnaught set is my all time favorite. This is the version that is no longer in the game. This is the best version. It makes me a sad panda. I have both versions of the shoulders from the newer Naxx, but I am disappointed that I was never able to atain this version. If they would let us recolor our gear, this would not be and issue. I much prefer the blue accents to the yellow ones on the new gear.
I have several other favorites that I will post below based on my characters.
Warlock tier 5 is certainly my favorite Warlock set...by a long shot.
For my Paladin, I prefer their tier 2 set.
For my Druid, I really like the tier 7.5 set the best. I like the tier two helmet (which is what I am wearing), but I prefer the 7.5 set for Druidy goodness.
And for the pries, I think my favorite set is tier 5. It just looks AWESOME.
So, those are some of my favorite outfits. What are yours?