out of all the villiains who would kenshin lose to

There is also the fact that Kenshin refusses to kill anymore. Meaning that his opnets get the chance to study and learn ways to beat him.

 

perhaps, but while he doesnt kill his opponents he destroys them in other ways. case in point, Isurugi Raijuuta. although Kenshin didnt kill him he defeated him in a way that destroyed his pride as a swordsmen. even with the ancient texts of the Izuna technique on his side, he had been beaten by man that could only use his left hand and hadnt been phased even when hit with the Madoi and Tobi Izuna. Raijuuta was perhaps far too confident in his Izuna technique as he almost soley relied on them in battle. in the end his false ideals that he held the "Satsujinken" were far too much and his realization that he in fact had never killed anyone was as much a breaking point as his weak form of Kenjutsu.

"The weight of a sword that stole a life will drag you down to hell. that is the killer's sword. if you do not understand that you cannot even beat Yahiko"

You also have to understand that while Kenshin may be now in full control of his Hitokiri side, it still is a part of him and while it doesnt take over his conciousness any longer it still is part of him in battle. in battle his Hitokiri side shows up in his prowess as a swordsmen. he does things in battle that a lesser man would never do. for instance, in the first fight against Aoshi he countered his Kodachi by grasping his sword by the blade, risking his hand.

as an interesting note about Raijuuta, he at one point starts with a Madoi Izuna and then immediatly as his sword touches the groud reverses it into a Tobi Izuna. this particular technique is called Tsubame-Gaeshi and was supposedly invented by the man known as Sasaki Koujirou although his existance was never proven.