Elite, 4-Star, Bloomer, and Hart. If you want to consider Cimarron and Platinum, they aren't too bad either. Sundowner is mid level, Circle J is low level. The above previous posters are ignorant to horse trailer quality.
Ifor Williams make the best horse boxs, both for the comfort and safety of your horse. There are others who are good. Always check the floor - ALWAYS lift up any mats/floor covering, to look for weakness in floor.