I just installed on my 1978 Euro delivered R65 new Heidenau Front K44 90/90–18 and Heidenau Rear K36 4.00-18. These tires have entirely transformed the bike's handling.
Previously it had Metzler Lazer. The bike handled like a tractor/ATV. Heavy and the transitions were full of notches. Now the handling is light and predictable. Transitions are smooth and notchless. Beautiful. AhHa, this is what BMW intended!
In 50 years of riding/driving I have never experienced such a huge transformation from a new set of tires.
I have never had anything but the Metzlers on the bike so cannot compare to any other brands. However, I have read all of the tire related threads on this forum in their entirety (up to a few months ago).
My conclusion is that the Heidenaus are the
only way to go. I am having difficulty expressing just how great a transformation this is. Would the much overused word "awesome" possibly convey? Go Heidenau.