What do we know about Itachi? We know that his eyes are inferior to Madara’s eyes, his Susanoo is not that strong because he doesn’t have eternal mangekyo sharingan, and he has the two weapons one of which makes you simply almost untouchable and the other can seal anything for eternity.
But of course Itachi’s stamina and chakra level are far less then Madara’s so he can’t use his Susanoo as long as Madara can which is quite a big dissadvantage.
We know quite something about Itachi form others – we know from Hiruzen that Itachi’s reasoning was like a hokage’s at the age of 7.
That says quite a lot, to think like a kage is a whole other level, which cannot be acquired by just a battle experience (which Madara probably had more than Itachi). Itachi is intelligent, which Madara is really not, even Hashirama is not really that much intelligent. Itachi was a pacifist who didn’t really have friends or enemies (after the death of Shisui, of course).
We have seen his briliant mind which had planned things years before they happened. And then there is the other side, which is more interesting.