Leaving out the Leotax G, which is outside my price range, I think it's kind of a tossup between the Leotax Merit and the Nicca 5L as "best Barnack clone" or maybe even "best Barnack (heresy!). They both have wind levers, both are the size of an M3, and both have excellent build quality, with maybe an edge for the Leotax.
The Leotax also has a self-timer, which the less-is-more folks shun, but I like it. It comes down to the Nicca's hinged back vs the Leotax's large bright finder with 50mm framelines. I have used both cameras, and my vote would go to the Leotax.
I just picked up a 5l and have the Merit too.
The finder on the Merit is nicer but once I tried the Leica 1:1 SBOOI finder on the Merit it is hard to go back. With that the finders become a wash.
A couple of other differences I have noticed.
The self timer on the Merit is nice.
I like the count down film counter on the Leotax but it is fairly easily changed with the ring on the side and it is also very easy to partially rotate the rewind ring to the point that the counter is disengaged but the film still winds normally. This makes the counter less trustworthy. This has happened with every roll I have put through it. I think the more traditional counter with the rewind button on the 5l may actually work better.
Both have film speed indicators. My Leotax seems to change as part of using the lever wind, my impression of the 5l is that it may stay set better.
The 5l allows you to set the shutter speed correctly before or after winding. Can't do that on the Leotax.
The 5l has a faster max shutter of 1/1000 vs 1/500. The Leotax FV has 1/1000 of a second along with everything else on the Merit.
I don't like rangefinder focus adjustment on the 5l. To easy to hit it and make the viewfinder blurry.
The Leotax has lever rewind which is quicker. The clutch on mine sometimes slips during rewind though so it may need some work.
Aesthetically I think the 5l is slightly cleaner looking with the hidden screws on the top cover and some of the dials are more polished than on the Leotax. I also like the way the body builds up to the high speed shutter dial compared to the stair step front of the Leotax.
Haven't shot the 5l yet though. Slow speeds are shooting fast so the camera was dropped off to Youxin Ye for a CLA.
One other difference I found. Leica FILCA cassettes will not fit in the Leotax at all, they do fit in the Nicca. The Nicca's unlock tab doesn't unlock the cassette though, you need to unlock it and turn it slightly before putting it in the camera. If you do that the door latch will open and close the cassette. I haven't loaded one yet to shoot though.
Shawn