Cruel, composed and bland
I always understood Mario Sunshine's King Boo as not the real thing, but rather a 'fake' King Boo made out goop - which goes to explain his unusual appearance, and why this design was so unique. Sunshine design could be used as an alt for Mario Kart Tour though.Of the King Boo renditions, I would've loved it if the Sunshine's King Boo is used more. Mario Kart's King Boo (the most commonly-used version, in fact) is rather plain, Luigi's Mansion's King Boo is better off in its own series than a wider one.
I have a certain fondness for how unique this version of King Boo is, with its tongue sticking out and relaxed eyes, which is more or less how the Boos in Super Mario Sunshine are portrayed. This version would serve as a reminder of the unique flavour of Super Mario Sushine, much like how both Petey Piranha and Gooper Blooper celebrate the Sunshine variety of those enemies, instead of going through a redesign to make them closer to the standard versions of the enemies (basically what King Boo is: basically a Boo but bigger and has a crown).
Thank you for reading.