I own a couple of Leotax's now and use them from time-to-time. They are not as well made as any Leica Barnack I have seen. They are however really cool cameras to own and use if serviced - lots of fun. The Leotax F is rather common and not very expensive these days.
The early Nicca's are pretty impressive judged by the one or two I've had, but I don't have that much experience using them.
In my opinion (and I'm sure I'll take some heat for this!), the Canon Leica copies are the best of the copies overall - really damn good examples of that genre, with some superior features such as switchable finders for 50, 100, and 135mm. Many Canons have only seen seen light use, which is great for todays collector/user crowd. The Leica's are still better made - built by hand like vintage Swiss watches, particularly some models, like the II series (and maybe the I series, of which I have only limited acquaintance). The surviving Leica Barnacks have typically seen a fair amount of use as working cameras (but most have held up).
All that said, the Leica IIIg, essentially a Barnack, is an unrivaled opto-mechanical 35mm masterpiece, deserving a place in the museum of camera greatness right next to the early double stroke M3's.
