I have R4, R4s, R5, Leicaflex SL & SL2.
The R4 & R5 models have bright viewfinders, spot metering and well-dampened shutters. All in all solid, nice compact cameras. They do have a little shutter lag (which the Leicaflexes don't have, being almost rangefinder-like in that respect), but no worse than many similar cameras.
The Leicaflexes have even better viewfinders (possibly the best in any 35mm SLR), fantastically dampened shutters (again, slow hand-held speeds are comparable to using a rangefinder), spot meters and... feel wonderful!
This at the price of extra size, weight and no automation.
The Leicaflex SL can't use some lenses (mainly later zooms and wides), while the SL2 can mount everything.
Some early R4 samples had electronic problems, which forever gave the model a bad reputation. Such problems have by now long been fixed and I've used 2 R4 bodies with *bad* serial numbers for years with no problems at all.
The R5 offers a brighter focusing screen (which you can also put in the R4), 1/2000 and adjustable program mode. I got it for the 1/2000 (for using telephotos), otherwise the R4 is just as good.
Many rave about the R6, but I don't see any advantage over the Leicafelxes, apart from being smaller, lighter and newer (not always automatically good things).
Finally, choose your lenses carefully.
While Leica made very few *poor* lenses, some are far better than others and signatures vary considerably. A little research will definitely be worthwhile.