It's really time that people start getting over the fact that it is Obito. It's Kishimoto-sensei's manga, so why do people think their theory of Tobi is better than that of the mangaka himself?
He clearly stated in an interview that everything has been planned out already, so Tobi being Obito was already decided. Kagami and Izuna were only mentioned a few times in the manga as opposite to Obito, who was mentioned a lot. Kagami and Izuna don't have any reason to start a war, Obito does.
You may call it the most obvious choice that Tobi was Obito and therefore it isn't a good choice, but funnily enough a lot of people thought it too obvious so it should be someone else. I like the plot twist that Kishimoto-sensei put in that.