I don't know about you guys, but Leather seems to have the most terrible looking designs in my opinion. It's a tough armour-type to transmog for, that's for sure... I've been unable to find gear sets I'm 100% happy with for both my Druid and Monk, yet my Paladin has 10 or more different sets I'm extremely happy with and can't decide from, with my Priest not far behind. My Shaman seems to have a bit of both of the Leather and Plate diseases, by having a mix of very disgusting sets (T5) and then some really amazing looking ones.
Maybe it's just me, but I feel Plate has the best overall look, with Cloth not far behind followed by Mail and then the bottom-of-the-barrel Leather.