1st off they're both really powerful ninjas and they both can beat each other but it all depends on information. For example, Jiriaya could of beat pain if he knew pain's secret, pain admitted this himself. Now in this fight you can tell Itachi didn't know Nagato's abilities and to beat Nagato you have to know them. After Naruto explained about Nagato's attack and seeing them for himself he made a great stategy. Now in a 1 on 1 Itachi would have to either already know Nagato's powers and weakness and could use genjutsus, and yes that is Nagato's weakness visual and all. The sound genjutsus was all Jiriaya and the toads had since Jiraiya isn't good at it so Itachi could win with genjutsus. But if Itachi doesn't know Nagato's power he would have to use shadow clones to see what Nagato can do, using exploding clones, hidden mist jutsus, and sealing. Itachi can seal all of Nagato's summons, use hidden mist jutsus, shoot a fire ball at Nagato,as Nagato dodge or absorbs it Itachi can strike him with his sword from behind or the fire ball can be a clone since Nagato can't absorb clones (seen when Naruto used rasen shuriken against pain and it was really a shadow clone) then place Nagato under his strongest genjutsus and kill him. Nagato only uses the seven paths so Itachi would find his weakness but if Nagato used other jutsus with fire, water, wind, earth, or lightning then maybe just to switch it up a bit ( tho Itachi would be able to copy and use it too). Oh and Nagato should be able to use izagani (spelling) like Danzo and Madara, even without the DNA of the uchias. Madara stated that Nagato was the third sage of six paths also claiming to be the 2nd back when he found Nagato's body. So the can do anything the sage can do just now as powerful.