The third/ last set of them looks the best in my opinion.

Made a better Bowser head:
   
The reason why either of these heads are bald is because I'm planning to add hair to them.

Shrunk my better custom Bowser head's horns and gave it hair:
   

Fixed a pixel in my better custom Bowser head and made a better body for it:
   

Better! Better! BETTER!

In all seriousness, it looks good so far.






...better.

Finished my better custom Bowser by giving him arms, legs, and a tail. He is here:
   
What's the difference between my custom Bowser sprite from 03-12-2020 and my better custom Bowser?

Fixed my custom Bullet Bill sprite by making his arms shaded like my custom Mario sprite:
   

Changed my better custom Bowser's color palette from 17 colors (counting transparency) to 16 colors (still counting transparency). As such, the shading on his bracelets was removed:
   

Made a Luigi, but it's no ordinary Luigi. It's Luigi from the "Mario's early years" games:
   

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Made "scuttling while holding a item" poses for my custom Luigi sprite. They are here:
   

Fixed the 3rd frame of my custom Fire Luigi's "scuttling while holding a item" animation:
   

Made Donkey Kong:
   
   

Made "front" and "back" poses for the Luigi seen in the "Mario's early years" games:
   

Made a custom sprite of Diddy Kong:
   

Made a better custom sprite of a Toad:
   

Improved my better custom sprite a Toad, added more colo(u)rs for him, and improved my custom Diddy Kong sprite: