[Specter]

I would have to say either Decade or OOO- because Decade can summon several ultimate riders at once VS OOO can become all his various forms at once- so with Decade and OOO you've got multiple riders involved in their attacks = you can do a lot more damage that way.

IF Decade Complete were to fight OOO, unfortunately I think Decade would lose- because Decade can only summon ultimate riders when he's performing attacks VS when OOO splits and becomes all his forms, they can pretty much run wild as long as he wants- so they could outnumber and corner Decade easily.