Nagato low-diff. Now that we've seen the Rinnegan has genjutsu capabilities, Itachi's best shot at Nagato is Susano'o, which is restricted. All elemental jutsu are absorbed, including Amaterasu, meaning the only thing Itachi is left with is taijutsu and his weapons. I'm really not sure about the genjutsu thing, though... Nagato could get out of 3T genjutsu, but Tsukuyomi leaves me uncertain. Itachi said only people of his bloodline were able to break free from it, and it was proven when Sasuke broke out of the illusion whereas Kakashi couldn't. I'm not sure about Tsukuyomi, but if Nagato can avoid it he takes the battle easily.