Having owned multiple copies of each, based on actual user experience (800+ rolls), the one to get, in terms of practicality in the field, and joy of use, is the Leicaflex SL. Each of the three versions has a clear, smooth, dimensional viewfinder, with a big 0.9x finder, and plenty of eye relief, but the SL has this crazy micro prism screen (fine micro prism circle surrounded by a field of coarse) where the subject just pops into focus in a dramatic way. Focusing is as much fun as getting the picture. Very easy to focus; quicker then autofocus even. The stock SL2 uses a comparatively boring split image, and the original has a central micro prism dot, analogous to a rangefinder patch.
The SL2 has a known defect at higher shutter speeds 1/500+ which results in blank frames. Depending on the technician, some claim it is a design defect that can only be temporarily fixed (Sherry), and others claim a flushing and relube permanently fixes the problem (DAG).
Sherry, by the way, is the premier repair technician for this model camera. Apparently she purchased all the spare parts from Leica USA. She's repaired many of them for me. Expensive to fix, but spare parts are available. Not many old cameras can make this claim. Sherry will tell you to get the SL. She calls it 'the M3 of the Leica reflex.'
They all have this effective braking system that allows shooting a stop or two slower then the competition. Not vibration free, but less shake then other SLRs. Not unusual to grab sharp images, hand held at 1/15. Very cool.
In terms of meter sensitivity, the SL2 is 10x more sensitive then the SL, and the original (non TTL) is in the middle. As a practical matter, the SL meters down to hand held levels. Hasn't been much of an issue in the field, but I'm usually out an about during daylight hours.
The original is the smoothest. It also has the brightest viewfinder.
They are all heavy (about 745 grams for the SL & SL2, 845 grams for the original) but actually lighter then much of the competition -- Nikon F & F2, Canon F1, ect. Lighter, smaller bodies were around the corner -- e.g., Nikon FM -- but that came at a cost of a smaller, squinty viewfinder.
I've owned them all, but quickly settled on the SL. Used them almost exclusively for 6-8years, burning through 800 or so rolls of film. Still have a couple. Just got tired of using the same camera all the time, but every time I pick one up, I think, 'This may well be my favorite camera.'