MJ,  There is no one best.   There are lots of choices and depending on how you will use the trailer, there are features that may mean more to you and others.

Consider the following and then go look at few.

Your Budget:  How much can you spend.

Size of the trailer:  Two horse, three horse, Four horse or larger.

Type of pull: Gooseneck or Bumper

Living Quarters or not:

Start by making a decision on some of these choices and then do a search in the trailers for sale area.   Once you have narrowed down to that list of available trailers that fit you budget, size and pulling requirements, then ask for suggestions on one of two of those brands.

Aluminum vs Steel or some combination of the two.

Fully enclosed or Open sided.

The list goes on and on.

Painted Horse started right on this.

Best horse trailer? You will get hundreds of different answers.

Priced reasonable? Again, your definition of reasonable and mine may be the same, or they could be worlds apart.

Here are some questions... Steel, aluminum, or aluminum-skinned? How many horses? Slant load or straight load? Bumper pull or gooseneck? Dressing room or living quarters? New or Used?

Edited to add:  What price range do you consider reasonable? This is the most important question. Some of the other questions may be answered automatically after this one.