I think Itachi because like he said, every jutsu has a weakness and he has shown that he is able to counter a wide variety of his attacks. Itachi also has his susanoothat he uses sparingly because of its side effect of causing blindness eventually, since the conditions state that Itachi is at full health then that means he could use his susanoo for a longer duration then he did against Sasuke+Orochimaru. Another reason why I think Itachi has the upper hand is because Pain appeared to be weak against genjutsu when fighting Jiraya, there were 3 peins attacking Jiraya and they were all caught in the genjutsu and were killed (but soon after revived since Jiraya didn't understand the paths of pain). Now Nagato does not have his paths of pain anymore, instead all the abilities are in his main body (Nagato). Which means if he gets caught in genjutsu then its over. The summoned animals also share their vision with pain, since they share the same vision as pain then it is possible that they will both be caught by the genjutsu used on the summon. Also pretty sure that Itachi can seal the dog summon with his sword. If you think the gravitational pull will be able to defeat Itachi, I also think your wrong, so far Nagato has shown that he must be standing still to use that move, if you stand still when the Amateratsu is after you then you will be burnt to ashes, also I'm afraid that absorbing these flames is not possible, its said that they will never be extinguished until the target is burnt.