You named a lot of good ones lol. Shadow Man, Gemini Man, Crash Man and Quick Man are some of my personal favorites too. I think, to a lot of people, the robot masters were definitely the main appeal of the original Mega Man games rather than Mega Man himself. It was always exciting to see what new bosses they would come up with.
I really like Fuse Man from 11, Knight Man from 6, and Metal Man from 2, to name a few others. Mostly for their designs and powers.