So Janemba and kid buu meet they both have fat forms and they are pure evil so who is stronger
Imo it would Mystic Buu to put him down.
The problem I have is that Goku is not kid buu, it is as simple as that. Kid Buu can take any one of Janemba's moves, it took the combined energy of namekians, earthlings, otherworld and z-fighters to threaten his life.
Kid Buu was toying with goku the whole time, and also can produce devastating attacks, not to mention absorption being one of them. Plus if you want to compare battles kid Buu gave alot more pain to vegeta than Janemba.