As much as I like Itachi, I've gotta say that Pain takes this. All six Pains vs Itachi is a lose situation for Itachi, mostly because, as great as he was, if all Pains attack at once they will overwhelm him. He would have to dodge and counter the summonings from the Animal Path while Pain could hide away the Naraka Path (Ressurection) inside of the chameleon to keep it safe. Provided that he could take on all of the summonings the Animal Path has, which we don't even know the maximum number, he would have to deal with long-ranged attacks via the Asura Path (Weapons). He was shown to possess missile attacks, and could thread them through the fight and attack Itachi while he's dealing with the animals. And as shown in the Sasuke vs Danzo fight, Susano'o can be peeled away. To have the missile attacks, the animals, and Asura potentially rushing in with bladed weapons, even the Yata Mirror wouldn't be enough.
While all this happens, should Itachi try to utilize Amaterasu, the Preta Path (Absorption) could run around absorbing the jutsu, and while he might be only to absorb one at a time, whose to say they couldn't gather together and be taken care of at once? Or if a summoning gets his they can just be unsummoned and summoned again (I think) since in the fight against Jiraiya, the chameleon was sliced apart by a water jet and unsummoned, but is seen again later in the manga.
This isn't including the Human Path (Soul). If it lays one hand on Itachi, he's done. End of discussion. Itachi's soul will have been taken and he will die. The Human Path can be running through the ranks, combining his own attacks with the others and attempting to peel apart Susano'o to get his hand in and grab Itachi.
Now comes the real trouble, the Deva Path. Deva could keep his distance like was his strategy vs Naruto. Human, Asura, Animal, and Preta would do all the fighting while he sits back and aids when needed. If Itachi tries to utilize the Totsuka blade, he could simply push or pull it away, leaving Susano'o open to attack, and could do so again a mere five seconds later. This could also be used on the Yata Mirror to open him up for attacks.
Even if Itachi were to take out the five other paths, he will have exhausted most of his chakra and potentially strained his eyes by that point, leaving him vulnerable for Deva to come in and finish him off. If he himself didn't use too much chakra beforehand or kept some in reserve, could force Itachi into a situation by using Planetary Devastation. If Itachi can manage to destroy it, he will have used up his last bits of chakra and will surely be dead following.
Now, if he hits a Tsukiyomi on one of the Paths, I feel it wouldn't matter. The Pains cannot feel pain. They are dead. Tsukiyomi is intended to inflict pain and cripple an opponent, but that cannot work on them, and even if it does, they can be reborn from Naraka. If he attempts to get to Nagato himself by using it on a Path, Nagato could simply cut off connection with that Path and remove the link to his eyes.
As for Yasaka Magatanaba (Sorry if mispelled), Itachi never performed this in his life that we know of. Only when he was Edo did he use it. Plus we don't even know what effects it has officially. All we've seen it do is, with help, blow up the Planetary Devastation core.
So there you have it. There are the strategic means Pain could use to defeat Itachi. If you have a counter then say so and don't resort to fanboyism and just ignore all the facts I brought up because you will be ignored.