Susano'o Is heavy. Look again. He shattered the ground. It's just that CT is strong enough to pick it up. That dosen't mean Susano'o isn't heavy enough to endure a Shinra Tensei.
You think he would have time to absorb Amaterasu. After he uses Shinra Tensei, he has to wait 5 seconds before he can use another technique. So he'll be vulnerable for Amaterasu and especially Genjutsu which he'll always be vulnerable to throughout the whole battle.
Incase you hadn't noticed, Genjutsu basically disables the victim to use Ninjutsu, or Taijutsu. Since it can paraylize your body (Itachi did to Orochimaru). Or make you see illusions, without you even noticeing like Sasuke did Danzou, and Itachi did Deidara. Ninjutsu is useless when you're seeing everything the user wants you to see. And feeling everything the user wants you to feel.
Nagato's physical attacks (Ninjutsu) is useless when inside of a Genjutsu. (Especially Tsukiyomi). Which he cannot break since he dosen't posses the tool, nor Itachi bloodline. Like Sasuke does. And theres no way you can beat Itachi while looking at his feet, or closing you eyes. He's already to tough to beat with your eyes open, so closing your eyes while fighting a Ninja with his intelligence and efficeny with jutsu is almost suicide. Even if Nagato COULD escape a Genjutsu, it would take too much time, you have to concetrate your chakra, which would be nearly impossible, while getting stabbed, or tortured in ANY Genjutsu, not just Tsukiyomi. (As seen with Naruto, when he tried to escape. ) I gotta say Genjutsu > Nagato.
I only see the paths beating Itachi, but NOT Nagato.