My preferred system is
ReadyBrake by NSA.
I've had mine for well over 20 years without incident. It's completely proportional and requires no power. It connects very quickly and easily and disconnecting is just as easy. I use the breakaway cables too. I have a small LED bulb on my dash that tells me when the brakes on the towed are being applied. Additional cable sets can be installed on other vehicles in case you have more than one towed.
All that plus it's the least expensive auxiliary braking system available.