I don't think any MMA fighters below heavyweight besides Alex Pereira would be able to make even top 40 in any boxing division, but heavyweight is different due to the thinness of the division and emphasis on physicality.
My picks (not in any particular order):
1. Prime Stipe Miocic
2. Prime JDS
3. Tom Aspinall
4. Sergei Kharitonov
5. Derrick Lewis had he continued under George Foreman
6. Alexander Emelianenko if he was less insane
My picks (not in any particular order):
1. Prime Stipe Miocic
2. Prime JDS
3. Tom Aspinall
4. Sergei Kharitonov
5. Derrick Lewis had he continued under George Foreman
6. Alexander Emelianenko if he was less insane