In a versus thread it depends.
Obviously So6p> All and he created the Bijuus so they automatically go to him,but if;
Tobi Vs Madara=I'd say Tobi owns since he can use his Normal Sharingan and Chakra chains to control tailed beast.
Hashirama Vs Tobi/Madara=Hashirama owns because he seems to be able to perfectly control the TailedBeast.
Still though, its fair game,Bijuu ownership is subjective to the creator of the "versus thread", since all of the above, Tobi,Madara an Hashirama and are capable of controlling Tailed-Beast.