Yahiko would win without trying. If you remember he and Kenji fought eachother in the Samurai X OVA Reflection and Yahiko beat him easily. Yahiko fights without rage behind his blows he fights to protect while Kenji is constantly angry. So, Yahiko is more focused in his moves and is clear headed. Peace out.
i go with Sojirou, because he has mastered a technique that is able to over fast the god like spped of hiten mitsurugi style, and for kenji to beat it, must have the same ability and accuracy of his father, Himura, but then again, none of them would fight.. think of it as a friendship bonding, Sojirou was saved by Himura, Yahiko is friends with Kenji, and Sojirou owe his life to Himura aka Kenji, so three of the brats are like friends... wouldnt you all think so??