Buu.

[Ander]

Shapechange can make him look however he wants, so long as he has seen whatever he turns into beforehand. However, Shapechange doesn't last forever: if he gets into a fight, any major blow can revert the shift, and it requires concentration to keep up the form. Also, he couldn't power up or use any super forms while Shapechanged. So really all it would do is alter his appearance. If Mr. Buu were to see Super Buu, and decided that's how he wanted to look, that's fine. But it wouldn't be beneficial beyond aesthetics.

If you cannot roleplay a person without having them look a certain way, you shouldn't have picked them in the first place, because you don't really care about the character itself.