Moorabi is definitely the easiest. I dont like suggesting UK because people tend to get stuck on the first boss and its not worth the effort to clear all that trash for just 2 bosses. My guess at difficulty would be this:

Gundrak
Draktheron
Utgarde Keep
Culling of Stratholme (if you arent racing clock I would call this the easiest instance.)
Violet Hold
and I never really ran the rest more than once. Most were awful to boxers.