[KamenRiderOOO]

I might as well tell you my vote and reason, since you all know who I'd pick.

OOO
Reason: Let's see them fight in this scenario:

They first fight in their standard forms, then Decade switches to Kuuga then Agito to attack, before getting a bit cocky and switching to Ryuki to summon Dragredder then Kiva Bashaa and Kuuga Pegasus to shoot at OOO. OOO at that time switches into GataKiriBa to corner and attack Decade by multiplying. Realizing that he'll need to distance himself, Decade switches to Faiz Axel, then tries to run. OOO turns into RaToraTah and chases after him. The two then get into a high speed battle, before Decade realizes he'll need strength to fight OOO off. He'll switch into Kuuga Titan or Den-O Axe form and try to hit OOO, who'll run around and switch into SaGoZo. Using SaGoZo's gravity controlling powers, OOO will trap Decade's legs in the ground. As he runs up to deliver a devastating punch, Decade will turn into Blade and use Metal to soften the blow, then turn into Hibiki and breathe fire into OOO's face. OOO switches to TaJaDor and returns fire, resulting in an explosion. Decade will turn into RX's Bio Rider form and hide in the water in an attempt to do a sneak attack. OOO does the same by turning into ShaUTa and finding Decade via Shachi's echolocation abilities. He'll then use the Unagi Whips to shock them both, knocking them both into their base forms out of the water. Decade turns into Kabuto and activates Clock Up, but OOO goes Super TaToBa and stops time, negating Clock Up. Decade realizes he needs to go all out and turns into Complete form. As he takes off the K Touch to press onto a rider symbol to summon back up, OOO turns into PuToTyra and freezes Decade's arms and Legs, before finishing him off with a blast from the Medagabryu and a slash powered by all the cell medals he has in him.

If you're wondering why Decade doesn't just stop OOO's Combo Changes, that's because the transformation creates a barrier that shields OOO from attacks