In context with the fact that Tobi has Obito's body, his leading statements about his history with Gai and Kakashi imply nothing but him being Obito (not to mention obvious metafictional evidence like the fact that the chapter where Tobi's identity is revealed is called "Uchiha Obito").

The bigger factor here is that if this person is not Uchiha Obito, in mind and body and spirit, every piece of evidence leading up to this was all just misdirection and pointless. Whoever it might be, if it wasn't him, had zero buildup and was a noncharacter in the series. All the deeply personal pathos and the connection to Kakashi and Gai are ruined if it was anyone else. I can see if he got help or possibly even if at one point Tobi was another person, but it's all but confirmed now that he is and has always been Tobi.