Make whatever you think will sell best. Here's what I think about it.
Dragonscale: Not many folks want it. Blue dragonscale is the best, black is ok, green not so good at all.
Elemental: requires a LOT of essences, which are expensive, and hard to farm. Both Druids and Rogues will like this stuff.
Tribal: Mostly Druid items, though not all Druids like the +spirit items. Some excellent Rogue gear too.
Of the three, I think Tribal would be easiest to make and sell, though Elemental would also sell well if you can get all the essences. If you are on an old realm, I doubt you will sell much because there are a lot of folks who can craft these items. On a new realm, I think you would do well because many people want the armor, and few can make it. I know I am doing well with Tribal.