Frank, to be honest, I haven't shot enough to be able to tell. I mean, I'd have to carry them around all the time, shooting all three for several months to be able to provide a precise and argumented answer to that...
However, from my brief experience with the M6, I'd say that both the M2 and M3 are a bit "smoother" in operation (wind lever, etc.).
Also, I have not tried the M6 with the grip yet - both my M2 and M3 have grips, which might account for a small difference in operation. With the M6, I'm using Lutz's "Sling". Felt a bit strange at first, but it actually eliminates the need for a grip. However, due to the position of my fingers in the sling, I usually end up winding to the next shot with two short strokes on the lever.
But, that's not a real difference in camera design and feel - just that I use the M6 in a different way.
Still, I think I can feel the diffeerence in the operation of the lever - the older ones are smoother.
I'll probably put one of the grips on the M6 these days and then I'll be able to make a proper comparison.
One thing is for sure - the Leicas handle much better with a grip - at least for me.
Oh, and BTW - my photos have not improved with the M6 😀
Denis