I think they are all problematic. However, if it was one, then I say Tobi would be the problematic, Although he hasn't shown his true power. Madara could trample Tobi because of Kabuto's work via Edo Tensei upgrade. Also, I don't think Madara would count because he is part of Kabuto's weapon (Edo Tensei).
Kabuto's might have change the way he operates (Sage Mode), but can still be problematic thanks to Orochimaru's and the studying of others DNA.