Kelsey Hayes Hydraulic (very hard to find anymore because of anti lock brakes). It actually taps into the hydraulic line so it mimics the brake peddle. The Jordan Ultima uses a cable on the brake pedal to effectively do the same. They are the best on the market.
http://www.bestbrakecontroller.com/
They are proactive controllers. they mimic the brake peddle on the tow vehicle EXACTLY.
The rest are reactive controllers that use some sort of inertia sensor that reacts AFTER the tow vehicle activates its brakes. They can be tricked. If you loose partial brakes on the tow vehicle then the tow vehicle can't produce enough change in inertia to get the control to kick on the trailer brakes. With. The Jordan the brakes on the trailer would work regardless.
Same deal with slick surface. the tow vehicle would lock and the trailer would start pushing it til it jackknifes.
The best of the reactives is probably the
Tekonsha Prodigy.