Here are the boss rankings, note that you might not prefer my opinion on some characters, but please, respect it. As I have saw other people having their favorite characters, well, I wanted to do something different with my favorite & least likeable characters. It also includes the Casino bosses. Here are my opinions:

Place Name Image Opinion
Werner Werman Rat german Werner has shades of brown, one of my favorite colors on my list, and I also like his design, including the Cat's. So he becomes the Winner of my character opinion list.
Wally Warbles Wally1 The runner-up is here, I also like his design & used to be Number #1, but I liked Werner more than him.
Grim Matchstick Drag mor Should there be a name for 3rd place? Well, the question is yes. I seriously like the designs of the 3 finalists, but I prefer Wally & Werner more than him.
4th Cagney Carnation Cagney flower 2 The closest to the 3 finalists, I like Cagney's phase 1 design, but I hate his passive & second phase designs.
5th Dr. Kahl's Robot Robit I hate Dr. Kahl's design, and prefer his robot's design than him.
6th Mangosteen 8Ball I like his design, and well, 8 is one of my favorite numbers.
7th Weepy Weepy I rarely hate him crying.
8th The Devil Cuphead devil sprite I rank him on this place because some people hate him and I want to respect some opinions.
9th Moe Tato Potato I prefer Weepy than him...