What's the best way to visually compare codec compression?
I can only think of 2 perspectives: industrial and recreational.
Industrial: visual examination is super expensive, you may take a look of how Netflix do the comparison https://netflixtechblog.com/performance-comparison-of-video-coding-standards-an-adaptive-streaming-perspective-d45d0183ca95
Recreational: you may check out youtube with codec comparison e.g. 
                
             or doom9 
                
            
Hope this answers your questions.