This topic hasn't really been explored much but i think its time that we discussed it. The manga hasn't really given us a comparison, but it should be obvious who was superior.
Prime MS Obito
Lets assume Obito used an extra Sharingan for Izanagi (like he used against Konan).
- Tsukuyomi would be countered by Obito's own MS.
- Obito has dealt with Amaterasu before (when Sasuke used it on him for example).
- Susano'o would not be entirely effective since it can't hit Obito, and Obito could walk through it.
Looking at these points it's hard to imagine Itachi really being superior to a prime Obito.