I have to admit I'm not too creative with transmog sets... But I love the look of my gnome in Malefic Rainment. But you know what's even better than that? The same gear with a more awesome colour scheme! Vengeful Gladiator's Felweave suited the job well.
You can't see it from my armory snap, but I'm so glad I kept my old Soul Harvester from the Vanilla Warlock mount quest. Looks awesome
Shame Blizzard moved to different art styles for the raid/PVP tiers... I loved the BC approach.