Unbiased suggestions? I'm not sure there is such a thing as an unbiased suggestion but here goes...
I don't believe there is a perfect supplemental brake system. Every one of them has something about it that somebody else thinks another product handles better. One popular system is the NSA Ready Brake/Ready Brute. Blue Ox also makes a similar designed brake system they call Autostop. These systems operate using inertia to operate a cable attached to the towed vehicles brake pedal. Simple and effective, and I've read few issues associated with this design. This design is one of the easier systems to hitch up once it is installed.
There are several manufacturers of systems that essentially sit on the floor of your towed vehicle and operate the brake pedal using inertia sensors, and in a few designs air pressure for brake operation. An arm must be attached from the unit to the towed vehicle brake pedal every time you want to tow.
There are what I'll call hybrid systems that utilize electronics and activators that while design wise are complicated, operate in what most folks consider a proportional manner. Invisibrake, Brake Buddy Stealth, and US Gear's Unified Tow Brake fall into this category ... there are others.
A lot depends in my opinion how often you tow, and in what conditions. Will you be hitching/unhitching often, or is your plan to tow infrequently, and especially will you be towing different vehicles?
If you will be hitching and unhitching often, I suggest looking into a system that is easy to hook up. A supplemental system that requires routine installation/removal of the brake actuator arm inside the towed vehicle may not be your cup of tea when it's raining or snowing. If you will be towing in icy conditions, you may want to consider a system that allows independent operation of the towed vehicles brakes.
Now for the biased part... I now use US Brake's Unified Tow Brake. After over 100,000 miles of towing a Jeep Wrangler of one sort or another and a couple previous different brake systems, I've chosen it because of it's ability to operate the towed vehicles brakes independently of the towing vehicle for slick roads, and after installation there is nothing easier to hitch up and go.
Here is my review of the UTB if you are interested...
Unified Tow Brake - Review