The EF 500mm f/4L IS II USM is scored 25 and EF 600mm f/4L IS II USM scored 26.
Compare to EF 400mm f/2.8L IS II USM:
Both 500mm and 600mm lenses are high end Canon L series lenses. Built with fantastic quality. If you’re looking for the best, these should be at the top of your list. The con point is that they are both heavy lenses and they are also very expensive.