I'm basing my results off of the only fight those two had against each other, minus B and Naruto.
facts:
From the manga we cannot discern the effectiveness of genjutsu against the rinnegan.
We know that Susanoo is affected by CT, it was getting drawn in.
Nagato could absorb chakra directly from the body as well as projectiles.
Amaturatsu can be repelled by ST.
Itachi posesses an eternal genjutsu via sword thingy
speculations:
Nagato can seal the souls via human path and hell god thingy.
Nagato can see through his summons eyes.
Nagato can absorb all forms of chakra, he can absorb rasengan and directly from the body, so it's safe to asume all forms of chakra can be absorbed, including amaturatsu and susanoo.
taking these things into account I would say that nagato should easily win this one.
susano either gets absorbed or his attacks get repelled.
amaturatsu, same thing.
genjutsu's, may not work, since the rinnegan is a higher lv eye than the ms and even if they did seeing through the eyes of the summons would make it pretty easy to see that it is tsukiyomi or other genjutsu's being activated.
this already eliminates itachi's strongest movesets, everything else that he could possibly do is pretty basic ninja stuff.
this fight essentially boils down to one man, itachi, using his bare hands, a few tricks and a sealing sword to fight a guy, nagato, with the power to move objects with his mind, absorb energy and steal souls.
now, unless itachi can move like might or rock i don't see him winning against nagato in a solo match, hell i don't really even see him winning against nagato with the help of naruto and B.
from what i can tell in the manga nagato/kabuto completely ignored itachi and focused on naruto and B. then at the end, rather than using the ST to repel the sword he just stood there.
why do i think that? looks like he gained his mind back when there was still a cloud of dust and just took the sword knowing it was the only way to stop him. seeing as he was trying to help Naruto and b that whole time.
SO... NAGATO --Easy to medium