Majin buu was strong enough to take Goku and vegeta to their limits and even forcing them to fuse to vegito. Despite his looks he is one of the strongest enemy the z fighters have fought in Kai.
Janemba was from the fusion reborn movie. He strictly forced Goku to go ssj3 even in his baby form to beat him and when he transformed into super Janemba Goku had a lot of trouble and same with vegeta forcing them to fuse into gogeta who then beat Janemba. I want to believe gogeta is also stronger then vegito and since Janemba can teleport and warp time giving him the advantage over buus speed and destruction. But what do you guys think ?