Ok, since for some reason Nagato vs Itachi is back, I'll explian, once again, why Nagato is the majority winner.
Let me also explain the scenario. This is with each at their theoretical peak when alive, without plot related handicaps (so for me, I count it with Itachi NOT being sick, and with Nagato having the use of both of his legs).
So I'll just start out with all of Itachi's jutsu abilities, and explain how Nagato can counter them, and why it is more probable for him to counter them than not.
1st of all there is Amaterasu. That one is easy, seeing as how Preta Path just goes and absorbs it.
2nd we have Tsukiyomi (any other genjutsu would not be powerful enough to bind Nagato). I shall agree that in my opinion, Nagato cannot win if Itachi uses Tsukiyomi on him. HOWEVER, Nagato knows Itachi is a prolific genjutsu fighter, and would avoid eye contact (this is strategical, and very probable). To continue fighting, Nagato could close his eyes and fight with his sensing abilities, or he could fight using shared vision with his summons. He has shown to be very good at fighting without looking at his opponent, so this is entirely possible.
Then the last of Itachi's main arsenal is Susanoo. The Sword of Totsuka is the main problem here. However, Nagato is not stupid enough to let him get pierced by a full on stab so easily. Nagato is fully capable of flying out of reach, either by using Deva Path, Asura Path, or by using one of his summons. Also, we know that Nagato has sensing abilities and is also fast (in terms of reactions), so he also may be capable of dodging the blade.
So basically the ways for Itachi to win are to stab Nagato with the Sword of Totsuka or catch him in a genjutsu, both of which I just shown to be less likely to happen than not. And at the very least, Nagato has counters to both of them.
However, against some of Nagato's jutsu, Itachi has NO counters.
CT is an instant win. Regardless of when Nagato uses it in a fight, if he does, he wins. There is no way to escape.
Knowing Nagato's power, Itachi is FORCED to keep up Susanoo. If he doesn't, Bansho Tenin can suck him in and he would get impaled by an Asura path blade, or get his soul sucked out. So Susanoo is a must.
This leads us to the fact that Itachi's stamina would drain ridiculously quickly. Nagato honestly just has to fly out of reach of the sword and close his eyes, meanwhile he will shoot missiles at Itachi, and he would win by wearing Itachi out. No way for Itachi to survive there.
Chou Shinra tensei is also almost a definite win for Itachi. It would blow Susanoo back miles. There is no way Itachi's Susanoo withstands that, seeing as how the ground beneath it would also be getting shot back. Itachi would die.
And then there's Gedo Mazo...Nagato's 5 elemental abilities...etc. These all combine to give Nagato even more chances of winning.
So basically, overall, Itachi has 2 ways of winning. Both of them can be countered, and they both aren't likely to happen.
Nagato has at least 3 ways of winning, and Itachi cannot counter any of them. Two of them are also probable.
So, overall, Nagato CLEARLY is the superior fighter. He wins 70/100 times.