Kai is the best fighter without powers among the ninja.
think about it, all of the ninja have kept their powers throughout the season (excluding when the ninja sacrificed their powers for Lloyd, and Lloyd became powerless). The thing is, when the other ninja lost their powers, they were mostly in hiding. Kai loses his powers once per season (unfortunately), and he needed to learn to accommodate for that. Kai probably learned how to fight without his powers a lot more than the other ninja did. I feel like in the earlier seasons Kai relied more on his powers to fight, but in dragons rising he doesn’t seem to use them as much, and prefers to fight physically. I think this is because during the land of monsters, he gained new ways of taking down enemies and became immensely stronger (physically) because of the monsters he was up against. Of course when you put powers in the mix, Lloyd is probably stronger than him, but as for now, his physical capabilities and fighting skills outweigh him and the other ninja.