In my opinion a good trailer is one what is inviting to the horse. I prefer straight haul as it offers much more room for them. One that is open and has a lot of space, one that has a ramp with vents. One that is very accessible and safe. A trailer that has escape doors. One that will be able to accomodate various sizes of horses. I also look for lots of padding.
Also, one that will be stable on the road and not go from side to side when the wind picks up.