That is a very very difficult question, usually easily answered by how much money you can spend.
I started with an R4, then upgraded to an R5. I loved the R5, but needed to have speeds above 1/2000 if I wanted to shoot wide open outdoors. So I saved up and upgraded to the R8, which has a top speed of 1/8000 sec. I know it looks grotesque at first, but believe me, it fits very nicely in your hands (provided that your hands aren't "too small").
I've had the opportunity to use (not own) an R6.2; very very nice, but you are definitively paying big bucks for the difference between an electronic shutter and an all-mechanical shutter (which the R6.2 has).
The R5, R6.2, and R7 look and handle virtually the same, except that the R5 doesn't have mirror lock-up.
Anyway, I think I'm repeating info you already know.
I think, honestly, if money is not the concern and you like to shoot wide open (like I do), go with the R8/R9, which used may be about the same as a used R6.2, and in the case of the R8, maybe even a bit cheaper. If money is a concern, then go with the R7. It's a lighter camera than the R8/R9 too.
Or...you could flip a coin. Or buy two! (I had a Leicaflex SL2 as a backup for a while).
Oooohh: one major advantage that I can think that the R8/R9 could have over the R7/R6.2: the light seal for the film cartridge window. It deteriorates with age, of course, but you won't know it until you see a light leak in your pictures. It did happen to me with my R4. Easy to fix; hell of an annoyance --- they should have just designed the R3/R4 cameras themselves...that's what you get...
