I favor Usyk but suspect Kudryashov would be his biggest threat. He also has good handspeed and combinations which you rarely see from him but the disparity probably as as big as people may think and that power....that power. You're right though Medzhdov and Nistor can definitely punch. Dima has actually fought a respectable level of opposition especially considering he had no legit management/promoter until recently. For a guy with 17 fights doing things on his own Suda, Cox, Mkwasse, JCG, Palacios is a nice lineup.
Not only has he held up well, he embarrassed them, except for I think his last bout vs that stocky Romanian dude who knocked out Anthony Joshua in the Ams
Ya Nistor has that Frazier/Ibragimov hyrid swarmer with a sluggers power thing going on. Hardest fight i can remember Usyk being in.