Nemesis
Kenshin has faced numerous people, assassins, mercenaries, common footpads, lunatics bent on revenge and world domination, etcetera, etcetera. He has triumphed over each and every single one however, though not always with graceful ease. Let's count off all the major face-offs here. There's Kenshin vs. Saitou Hajime, Shishio Makoto, Seta Soujirou, Shinomori Aoshi, and Yukishiro Enishi. All five of them are really skilled fighters and most of them had a really good reason for wanting to do Kenshin in (except maybe for Soujiro). But motives are not really important here. What matters is how much damage each of them managed to inflict on poor Ken-san and who I think could have really killed him had they been given the chance.
The obvious answer to this, which I'm sure many of you are now yelling out, is none other than Kenshin's replacement in the Isshin Shishi, Shishio Makoto. I'm sorry but I'd have to really disagree. Why? Because Shishio is a cheater. Yes, you read right. He CHEATED. Horribly in fact. During the finale of the Kyoto arc, Shishio sent Yumi to "guide" Kenshin and his friends through the caverns of his secret mountain hideout. In reality, Yumi was there to watch Kenshin's moves and report to Shishio. This struck me as totally unfair because while Kenshin was completely clueless about Shishio's fighting technique, Shishio knew his and could therefore prepare himself for Kenshin's attacks beforehand. In his second fight with Aoshi in Houji's study, Yumi told Shishio that Kenshin's "ultimate" technique was the Amakakeru Ryuu no Hirameki but she unfortunately could not see it. However, right after fighting Aoshi, Kenshin ended up against Soujiro. This time, he used the technique once more and left the defeated boy behind. Soujiro then told Yumi the secret of the Amakakeru Ryuu no Hirameki, who naturally then told Shishio! Now you see how truly disgusting that is? Kenshin's most secret technique was revealed to Shishio, who now could use that knowledge to find an opening. Not only that, but Shishio used Aoshi to exhaust Kenshin for their final battle, a ploy that even Aoshi realized later on. Had Kenshin not had to fight his way to Shishio, perhaps he would have been more alert and stronger and would have hurt Shishio more before he went and spontaneously combusted.
Shinomori Aoshi was another "bad" guy that gave Kenshin a beating. In fact, he just came short of killing Kenshin because he was interrupted by an outside force and had to escape. But their second fight finally settled for once and for all who the better fighter was and that was obviously Kenshin. Somewhere along the road, Aoshi lost his way and became the empty and cold-blooded man we all know and love but inside him, the "old" Aoshi still lived, the one Misao loved, and with that part of him, he never became "evil" enough to defeat Kenshin...Eargh...It's a bit hard to explain. Let's just say Aoshi's heart wasn't really in it so in the end Kenshin defeated him, because Kenshin was determined to get back to Kaoru and his friends.
Now let's look at Soujiro, Enishi, and Saitou. This is the hard part. Soujiro was a truly gifted fighter and I believe he stood a much greater chance of defeating Kenshin than Shishio did (if all had been fair). This was due to the fact that Kenshin relied heavily on his ability to read his opponents moves. Because Soujiro "had no real feelings" this made it much, much harder for Kenshin to read his and Soujiro was beginning to really hurt him -- until Kenshin's silence and determination finally got on his nerves and he began to remember his past. It's hard to say whether Soujiro was stupid or mentally imbalanced. I'd say he was more on the insane side...Anyway, once he started to feel and act like a human being again, Kenshin figured him out and defeated him. Though Soujiro came close, his instability, which is an important part of his personality, made him a slightly lesser fighter than Himura. Very, very slightly less. This leaves us to choose between Saitou and Enishi.
If it comes to motivation, Enishi was much more strongly motivated, because his quibble with Kenshin was of a much more personal nature. Truly, he was a formidable fighter but, like Shishio, I think he used far too many tricks. Again, he went with that "go through all my sidekicks first before you fight me" routine. So annoying. This also gave him a chance to watch Kenshin and observe his fighting style and to "kill" Kaoru, which totally disheartened Kenshin. But of course, Enishi later learned that that had been a tactical error because once Kenshin recovered, he became one extremely determined fellow and proceeded to whip Enishi's butt. Not even Enishi's mind-boggling, nerve-popping, totally gross nerves of insanity trick could help him and in fact only served to bring him down. (More points for the Hiten Mitsurugi School! Who'd have thought an attack could be done that way?) In the end, Enishi was not quite a match for Kenshin, especially after the realization that revenge was not the way to put his sister's soul to rest. Remember, folks, there's a lot more involved in these fights than sheer physical prowess. Sometimes the psychological battles are even worse.
So now you must have guessed who I've got in mind as Kenshin's worst enemy. Yep. Saitou Hajime. Is he really Kenshin's worst enemy, though? We may never know unless we ask Nobuhiro Watsuki himself. He neatly avoided the issue, that...awfully devious manga-ka. The first time Kenshin and Saitou had an all-out battle (during which Kenshin reverted to "Battousai" mode), they were interrupted at the exact moment that they were about to deliver their finishing blows. Had those blows been delivered, one--or both--of them would have certainly died. In the final volume, Saitou turned down Kenshin's invitation to settle everything between them once and for all. Saitou, having fought Kenshin in the past and having stood witness to most of Kenshin's battles, knew Kenshin perhaps better than any of them did, for he knew both the person and the fighter, his weaknesses, his techniques, his habits. In fact, Saitou often took Kaoru's place as the one who explains "why Kenshin did this and that" during a fight. If they really faced each other in a serious, all-out battle, it is truly hard to say who could have won, particularly since both of them lived through the Bakumatsu and have equal cunning and quick wits.