Itachi, with reasonable effort. Reasons why listed below:
1) One of the most intelligent characters in the Narutoverse. An example of his intelligence would be him giving Naruto the bird with Shusui's eye incase Sasuke attacked the village or when he found a way to counter Pain's meteor move thing. That jutsu was one used by the Sage of Six Paths himself, albeit on a much larger scale( the moon); the fact he found a weakness in that jutsu is a testament to his intellect. Itachi's intelligence clearly trumps almost any character's we've seen, let alone Danzo.
2) Jutsu at his disposal. His most powerful eye tech (tsu...) will just confuse the crap out of Danzo. Susanoo and Amatersau will just burn the poor man. Dont forget Itachi's Susanoo has the sword and the Yata Mirror, giving him an ultimate defence and a bloody brilliant attack. His Susanoo trumps almost, ALMOST, any jutsu that we've seen so far.
One area I can say Danzo is better is stamina. If Itachi was ill during this fight, Danzo defo has the more stamina and MAY outlast Itachi, but a fully healthy Itachi wipes the floor, yes, wipes the floor with most of the characters, let alone old man Danzo.