Why the "air quotes?". Are you looking for an actual trailer, or something like a trailer, that's not really a bike trailer?
If it's an actual bike trailer, the THULE Chariot line is the top of the class. Good frame and suspension design, and upfits for days, so you can add features or use it for multiple sports. New, they can cost as much as a good bicycle, and the market for used ones is pretty solid too.
Burley is the Toyota to Chariot's BMW. They're solid and dependable, too, but just not the features
Things to look for, on any brand, are 20" spoked wheels, as opposed to 16"s and plastic 'mags' A good trailer will also have the wheels at the very back, rather than under the middle.
Finally, if you buy used, make sure you get all the hitch hardware. Many brands have a bracket that stays bolted to the bike, and sometimes sellers forget to take it off.