Hi,
That depends. Which camera would you put it on? Do you need AF? Are you shooting a lot at full aperture?
The Otuses have excellent optical correction, they will outshine almost any lens at f/1.4. But stopping down to f/8 they will be limited by diffraction as any other high quality lens.
The Sigma Art 50/1.4 would not match the Otus at f/1.4, nothing does, but it would put ut a good fight at most other apertures.
Depends a lot what camera you put the lens on.
Let's put it this way. At f/1.4 I am pretty sure no other lens can match the Otus on a high res DSLR. You can put the Otus on Nikon D810 or Canon's forthcoming 5Ds. On the Canon it will beat the Nikon and both will beat the MFD lenses at full aperture. Stopped down to f/8 it will be much more even. On Nikon and Canon you would be able to focus using live view, you would not have that option on the Pentax 645D or the old Hasselblad.
Technically, you could put the Otus on a Sony A7r, but vibrations may rob a lot of sharpness in the 1/5 - 1/250 s shutter speed range.
This is based on tests I have seen. I don't have the Otus.
Just to mention, if you don't need the f/1.4 aperture and can live with the focal length the Zeiss 135/2.0 APO is an excellent lens. Would Zeiss make an 85/2.0 APO of the same quality it would be on my buying list. In my case I am speculating on Sony and Nikon coming around with 50 MP pretty soon, even if it seems they are not in any great hurry.
Best regards
Erik