If you value optical quality above all else, the Takahashi "Q" is is the best fast 500-600mm (in this case 530mm f/5) production medium format lens of all time:http://www.astronomieplus.com/modules.php?name=ProductList&op=BrowseItem&ItemID=1616
Here's the manual for the previous iteration, the "ED" version, since the accessory system for the "Q" and "ED" are mostly the same :http://www.takahashiamerica.com/manuals/FSQ-106ED.pdf
Note the Mamiya 645
and Pentax 67 adapters on pp 20 and 25.
Non plus ultra. Thing is, it's designed to be shot wide open (diffraction limited to a ridiculously small fraction of a wavelength at full aperture). If you want to shoot at smaller than f/5, you need to rig your own aperture masks to cover the front. I could live with that
; indeed, I've done so for shooting through lesser refractors.