Do you really want to know the best one (as in best made)? Are you going to go out and buy it just because you hear it is the best?
If so, go buy a new Leica MP. Be sure to get a lens or two, which will cost you just as much. $10,000 later, you'd better start making a good living with that camera straight away, or you (and/or your loved ones) are going to start kicking yourself!
If you want to get the best bang for the buck, or simply a nice quality rangefinder without getting into crazy new Leica territory, there are endless options, many of which are really good quality and affordable ones. You can find a lot of discussions on this topic in the APUG archives.
My suggestion is the same for anyone thinking about getting a rangefinder. First, buy a cheap one, in the sub-$100 range. Try using a rangefinder. If you are like most people, you won't like it, it won't be suited to most of the photography you do, and you will take worse pictures with it because it is harder to use. (SLRs took over for a reason.) Then you say "yuck," and resell the camera. No big loss. If you love it, and find that you could really use one for the work you do, then you have only spent $100 to gain this priceless knowledge, which you can probably get back by reselling the camera once you buy a better-quality rangefinder.
Good "exploration" cameras are Yashica Electro 35's, Canonets, Minoltas, etc. They are inexpensive, they will provide the "feel" of what using a rangefinder is like, and some of them have very good lenses too.
If you are certain that a rangefinder will be a useful camera to you because you have already used one, then in the $1,000 to $1,500 camera+lens area I would get a used Leica M3 or M2. In the under-$1,000 camera+lens area, I'd get a Voigtländer Bessa (R model if you want thread mount, anything R2 or later if you want bayonet mount). Those are the best all around tools for the dollar spent in the rangefinder world, IMHO.
