Sasori is the most underrated character, period. While Madara is overall stronger, I ‘m not confident he would win. Genjutsu is nullified, which means Sasori does not have to worry about staring at him, he easily could close the distance then paralyze him with his poison. Susano’o users need to breathe and a massive, air tight shield would surely guarantee suffocation. Also, Madara starts with a stage 1 Susano, with no legs, Iron sand could be utilized to grab Madara and pull him out while the rest of his puppets go to town, it only takes a small cut, you know, something which occur in, almost, every battle. Hell, even Gaara and Onoki almost took him out.
Madara has the biggest, most dangerous arsenal in the manga I can’t see many people surviving if he decided to go all out. The thing is he never does that… he is extremely arrogant his main objective is to get a reasonable workout in before it’s all said and done, an opportunist like Sasori would exploit that. Naruto is all about match-ups and, generally speaking, power is overrated.
Minato only ran around with a rasengan, yet he could take out platoons.