Pretty self explanatory, (try to give at least a cursory explanation).
My current ones would have to be "Orochimaru is the Haichibi" and "Orochimaru is the Jinchūriki for the Hachibi".
Partially because of how it came about, partially because it's never been an issue before because of all the evidence against it in the past.
By the way, if anyone didn't notice, it's an open poll without an expiration date, and it supports multiple answers.
@ kaom, I just put up some choices that are relatively recent/popular (in my opinion), if there's lots of theories that aren't listed here and seem to be more "popular", can you please edit them in (replace whichever you want)?