best rear disc brake conversion kit ?

Doctor Diff. Awesome kit, without question.

Many of the kits on the market use a rear caliper with too large of a piston which necessitates a separate proportioning valve to get the front/rear bias correct. His kit uses the Ford Cobra caliper which incorporates a built in parking brake which hooks right up to your existing parking brake. The Cobra caliper has a smaller piston and works great without needing an auxiliary proportioning valve.

Another plus is that the Doctor Diff kit is comprised of OEM calipers and rotors. This, to me is important for future maintenence.