Nagato can beat Itachi.

[HadouKage]

The only problem itachi had was defeating CT, and we don't know if he could have done it by himself. Amaterasu or Totsuka could take out the orb, but we'll never know for sure. It all comes down to that.

What is this blasphemy my eyes have been cursed to witness?

 

[Mitarashi]

Nagato is very under-rated for some reason... People believe Naruto, the main character mind you, beat him because he was stronger, WRONG.
He used Talk no jutsu, nagato had the strength to revive hundreds of people, he still had considerable power..
Rinnegan are just > MS tbh, Most of the jutsu have no negative impact, they don't cause fatigue, Deva path counters Amaterasu, He never HAS too look at itachi eyes because he's got 6+ pairs of them, so he could watch from any other side.

Itachi can't beat Nagato.