Echo...Marvel is the best, and expensive.
I have been told (by someone who should know) that Ciener makes the Kimber conversion, but I have not seen the Ciener up close. I know a couple of guys with the Kimber conversions, and they like them. With the Kimber conversion, the slide does not lock back after the last shot. It's designed that way. I have a Colt conversion, It's an older one, and a real quality piece to match my Colt Series 70 Gold Cup. I have never had any problems with it. They are on ebay all the time for $250-$300.
Better yet unless you plan to do Bullseye competition, take the money, and buy yourself a 22 semi-auto. For about the same money, you can get a Ruger MkII. It will shot better than the conversion, and you will have two guns instead of one. If you plan to compete in Bullseye, get the Marvel.