You know, I'm kind of wondering which characters are most difficult to fuck now. Piccolo's from an asexual species that spits out eggs, so I don't think he has any genitals. He also doesn't eat food so maybe he doesn't have an ass.
Majin Buu is a bubblegum smoke monster so he could do some kinky shit. And he's covered in all those little holes even in normal form.
Cell would be a challenge because you'd be working around his carapace.