[Switchblade]

This shouldn't even be a contest. Blade's King form calls upon the power of all thirteen Spade undead at once, giving Kenzaki such a huge boost in power that he can barely control it. Add that power boost into Kenzaki's trained fighting skills and Blade has one of the more powerful final forms in the Heisei era.

Kiva, by contrast, can't stack up. Technically, he's faster and more versatile, but he's not nearly as skilled of a fighter as Kenzaki. Odds are he'd end up getting in close with the Zanvat sword and there's no way in hell he can compete with Kenzaki blade to, well, Blade (awful pun, sorry). Plus the Zanvat needs a good fapping to get it really charged up and the time Wataru needs to do that is more than enough of an opening for Kenzaki to throw a Royal Straight Flush in his face.

In terms of power, skill, and personal bias (Kenzaki's one of my favorite Heisei Riders, Wataru is my least favorite ever) I have to give this to Kenzaki.

Frankly, Wataru's only chance here is to travel back in time and keep Kenzaki's parents from knocking boots.