Well, I have owned three different 50mm's for my Leica M's over the years. I started with the current Summicron as my first rangefinder lens. It was fantastic. Only thing I didn't like about it--literally the only thing--was that it didn't have a focus tab. I really like having a focus tab as it lets me pre-focus before I even bring the camera to my eye.
When I bought the 50 cron, I thought what I really wanted was the 50 ASPH Lux. It had a fantastic reputation, aspheric surfaces to reduce distortion, was a stop faster, and had a one year waiting list (give or take). I got in line. When it arrived, I sold the 50 Summicron to help fund the 50 Summilux.
The 'Lux was every bit as good as its reputation, but it definitely had a couple issues I hadn't thought about. First, it wasn't as well balanced on the M body as the Cron had been. Second, it was just heavy enough to be annoying, though not really a problem. Third, the focusing was not as silky smooth as on the Summicron (though it did have the tab I so wanted), and fourth was viewfinder blockage. There was just too much. Still, I lived quite happily with the 'Lux for several years. Below f/2.8 the 'Lux was a better lens than the 'Cron, but from f/2.8 onwards they were indistinguishable to my eye. Sure, I could see a difference in bokeh, but I didn't consistently prefer one over the other. Obviously, the 'Lux was a stop faster, but I found I rarely needed or wanted that extra stop due to the limited depth of field and the need to really nail the focus.
Recently I had an opportunity to purchase a 50mm APO Summicron used for a very fair price--right around your budget limit of $4K. It's really the best of all worlds from my perspective. In certain situations I can see the difference in resolution (though those instances are few and far between--I basically have to use Live View to get the focus just right, then stop down, make sure I'm using a tripod, etc.. It's got the focus tab I love, has silky smooth focusing, a very elegant twist-up lens hood (which is enough to protect the front element from accidental knocking around, but not enough to do a good job of glare protection), is fairly immune to flare, is small and light like my original Summicron, has no significant viewfinder blockage, and is fast enough for any of the images I want to take. It's really the perfect 50mm. It even works well on my SL where I find it is easier to focus wide open than on the M.
I have never wanted a Noctilux due to the size and weight issues and the limited real-world use (at least in my photography) for such a razor thin depth of field. I'm sure it's a wonderful lens, but it just doesn't appeal to me.
I have never owned any of the slower 50's such as the Summarit or one of the older lenses so I can't speak to them.
From my perspective, the 50 APO is the best 50 I have ever seen or used. It is small, light, flare resistant (aside from the initial defective batch), has impeccable image quality, fantastic contrast, good bokeh--subjectively--and handles really well. I just don't see how to improve it aside from maybe lowering the price.
If your budget or your sense of value won't allow the 50 APO Summicron, my second choice would actually be the regular 50mm Summicron. It is very nearly as good as the APO, has wonderfully smooth focusing, is small, light, not particularly expensive by Leica standards. It just doesn't get much better.
In general, I find the Summicron to be my favorite of the Leica lenses at almost every focal length. f/2 seems to be where you get the best compromise on size/weight, price, handling, speed, control over depth of field, etc.. Depending on your photographic style, your requirements could be quite different, though.
- Jared
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