I have the Eastern Beaver H4 relay kit and a headlight modulator by SDC (Signal Dynamics). My aftermarket turn signals require a load equalizer and I opted for the electronic version by SDC for that as well. I did my research on tail light modulators and will be choosing the one sold by Eastern Beaver.
The best tail light modulator is the one sold by Easter Beaver since it does not annoy those behind you by flashing every time you touch the brake, it uses some common sense. The flashing operates on the initial stop and doesn't just flash the same over and over as you creep forward at stop signs. SDC makes a solid product as well, but it gets annoying being behind a bike with one since it does exactly the same thing every time the brake is applied.
SDC's Head Light Modulator gets my vote due to it's compact size, waterproof construction, night time auto disable and high beam switch on/off feature. It comes on automatically during the day, but can be turned off by just flipping the Headlight Hi/Lo switch once. It is also 50 state legal.
SDC's load equalizer is my choice for the same reasons as their headlight modulator, water proof, size etc.