I have trouble choosing just one, but the following three are the front runners for being my favorite.
One of my favorites has always been the Fang Dragon. Something about the artwork for it in 3rd Edition just really caught my eye, and I liked the idea of a more physical dragon. The reason it does not solely occupy my favorite spot is due to the lack of a breath weapon. I like to play a lot of dragon blooded sorcerers; getting no benefit from your draconic heritage kinda sucks. Though, in 4th Edition, I believe Fang Dragons were turned into Gray Dragons and given a breath weapon, so I'll have to consider that more.
My second possible choice would be the blue dragon. I always liked the idea of a lightning breath weapon.
My third choice, and one which is a little more obscure, is the purple dragon. It was one of the dragons created by Tiamat's sister in 3rd Edition. The artwork for it in the Dragon Compendium was awesome. Note that I am not referring to Deep Dragons (which are occasionally called Purple Dragons.)