I'm sick of all these theories about who Tobi is. It's Madara. Not Danzo, not Mifune, not Izuna, not Obito. Here's why.
Danzo is Inari's "Hero-Father"-
Manga Fox: Naruto Vol.03 Ch.020: The Country That Had A Hero Online Manga Scans
Manga Fox: Naruto Vol.47 Ch.455: Bond...!! Online Manga Scans
Mifune is just a traditional Samurai with different hair-
Manga Fox: Naruto Vol.47 Ch.458: The Five Kages' Argument Online Manga Scans
Izuna had both eyes stolen from him by Madara-
Manga Fox: Naruto Vol.43 Ch.399: The Beginning of Everything Online Manga Scans
Obtio's entire body was crushed, not just his right side-
Manga Fox: Naruto Vol.27 Ch.244: Gaiden, Final Chapter: Heroes of the Sharingan Online Manga Scans
There you go everyone. Madara is Tobi as has been stated repeatedly throughout the manga.