The Hasselblad Zeiss 250f5.6 and 350f5.6 Superachromat lenses are considered to be among the sharpest lenses for medium format digital photography in this focal range. But they are extremely expensive on 2nd. hand market.
The Mamiya RZ67 Apo lenses 210f4.5, 250f4.5, 350f5.6 and 500f6 cost a fraction and can be found quite easily 2nd. hand in mint condition. Is their optical performance with the current 60/80 MP backs quite to the Hasselblad Zeiss Superachromats or to the Mamiya 645 Apo lenses 200f2.8, 300f4.5?
I have shot with the 250mm Mamiya lens both on film and a few times with a digital back. The 250 is a fantastic lens.
However for the best results you should also get the double cable release for shooting mirror up. The RZ has a lot of machinery moving around
when the mirror goes up. Actually it's quite amazing how well damped the whole machinery is.
You will also find that shooting with the RZ waist level finder will be great for critical focus.
Here is how good it can do even with film. This is hand held and stopped down to f8
Crop from the same shot.
Most of the RX lenses are very good. I did not however like the 180mm.
Two other really good MF lenses worth looking at are the Fuji gx680 250mm and 300mm
You can even get a stitch back from Kapture group and do two frame stitches with a digital back and see your composition in real time in the
large 8x8cm viewfinder. The loup viewfinder is fantastic for critical focus.
A 160 MP stitch with these lenses would be very hard to beat.
Here are a couple of shots with the 250mm with film.
Crop.. Focus is a wee bit off and is on the tip of the nose.