[KyoryuGreen]

I'd say Kazuya/Super-1 because of his Martial Arts knowledge combined with being a cyborg would make his difficult to defeat. Kotaro/Black-Black RX would be difficult too I mean he lifts a car unhenshined in Black RX. Yuusuke/Kuuga is the man of 2000 skills so he probably has lots of different fighting skills there. Takumi/Faiz being an Orphenoch probably might make him stronger than a normal human. Kusaka/Kaixa is part Orphenoch and was well-trained in lots of things too. Kenzaki/Blade being a Joker Undead by the end of the series and can't turn that off would be extremely powerful to beat everyone. Tendou/Kabuto because he's Tendou (Dear God is he both the Batman and Superman of the Kamen Rider verse?). Yaguruma/Kick Hopper is extremely well trained and fought the Worms unpowered and stuff. Wataru/Kiva is half fangire so like Takumi might give him some advantage. Eiji/OOO well by his movies he seems like a fucking badass with roundhouse kicks and other skills like that might make him very hard to take down. Date/Birth's wrestling moves and other skills I'd give him a good chance. Ryusei/Meteor from what I seen looks like a martial arts master too. Oren/Bravo because he's an elite special force members. Yoko/Marika because she could handle Oren. Takatora/Zangetsu because he's probably extremely well-trained out of his transformation too.

Honestly I think Kenzaki, Kazuya, or Kotaro would win.