You asked about the MOST RELIABLE.
That may not be the right thing to worry about -- as reliability in a plinker isn't as important as it is in a self-defense gun. I'd ask about ACCURACY...
Most of the kits on the market are made by Ceiner. Wilson, Kimber and others use them. There is more than one level of quality.
The BEST, in terms of accuracy, reliability or any other criteria, are apparently the MARVEL kits. They're often advertised in Shotgun News.
Bullseye shooters use them in competition, and they're real picky about accuracy. The Marvel kits come with some obscene guarantee of accuracy. (I don't remember exactly, but its in their ad -- something like sub 1" at 50 yards.)
They cost about $100 more than the Ceiners, but the difference is dramatic. Better than the Colt ACE kits, if the folks who have had both are to be trusted.