By far, Itachi. Then Madara.
I know Madara is stronger, but he just hasn't been hyped quite as much as Itachi. Itachi has this...feel about him. This force of prescence that makes one want to slap all the 'tards who think Jiraiya stands two shits of a chance against him. We've seen him on the losing end against Sasuke several times, but only due to very specific circumstances, which were obviously tacked on by Kishimoto so that Sasuke would actually put up a close fight. Even now, Sasuke's left exhausted and Orochimaru came out, attacking as a separate entity, thus making the match no longer one-on-one. So even if Itachi still ends up losing, he at least has the honor of having beaten a Sannin and nearly beating someone who was also around that Sannin's level in one fight.
Itachi is just not the kind of person that any one ninja can beat. Even Sasuke, with his grandiose PnJ, is currently helpless. Oro lasted three pages, yet again. Itachi is just so much hax, it borders on the impossible.