iMac Retina 5K, 27-inch, late 2014 - best macOS version to install via OCLP

What macOS version for iMac Retina 5K, 27-inch, late 2014 is best supported by OCLP? I am planning to - install on external SSD connected via usb 3, and - use migration assistant to transfer users & apps from the original fusion drive to the external SSD with the new macOS.

Any tips/recommendations is appreciated!

Author: @berktaftali