hmm...that's a tough one. I think Buutenks has this. He's practically indestructible with a never-ending power supply. He'd probably just start insulting Janemba (assuming this Buutenks still has Piccolo) and then give him quite the beat down.
I'm going to have to go with Buutenks on this one. While Super Janemba did give SSJ3 Goku and SSJ2 Vegeta a run for their money. As soon as Super Saiyan Gogeta appeared he was absolutely smoked. I think it would be a little close because Buutenks is definitely not Buuhan, but Buu's regenerative powers would wear on Janemba. Even if Janemba would remain mobile through his phase teleportation.