I'd say anyone below. My friend believes he is a H2H god but I couldn't really think of anyone that could take him out in a pure H2H fight, at least not anyone I didn't end up doubting.
Ken is a beast in the H2H department, like a complete monster with his powerset. He was Goku before Goku was created. Abilities he has are: Shuieshin, a technique that allows him to read his opponents style, copy it flawlessly and predict their own movements. This is first referenced in his fight against Shuu.
Hokuto Hyakuretsuken, which is basically 100 punches in like a second, or less that he usually ends with a special punch.
Hokuto Shinken is the style that all of this falls under, it's an ancient Chinese assassination martial art. He has many other styles under his belt due to his journey although it's rare for him to use them, one for being weaker than Hokuto Shinken, two because it's not really his style to just use other's styles.
Musou Tensei is broken, it's like a counter move. It has never been fully explained but say you try to punch him, if he doesn't want to be hit then he'll like "phase" through the punch and become intangible. It should be noted that he can't just stay intangible, or at least that he's never been shown that way. I also personally believe that the move will only be activated if Ken can perceive the attack and see it coming.
Forgot the exact name but some kind of dragon breathing technique, it allows Ken to use 100% of his body's potential and energy. Every time he screams and flexes off his shirt that's him using the technique basically.
Strength feats are pretty good, he's had some feats where he easily lifts a boulder which other fans have stated over 1000 tons, I say it's at least 100 tons, he's lifted huge ass boulders on two occasions. Guy stopped a train with just his aura. There are some others that I'd have to fish through.
Durability is immense as well, low piercing durability, he's tanked moves that could easily slice through steel though. Blunt force and punching is what he's most durable as.
Speed is insane, just utterly insane, the guy has tied cracking whips together before the opponent could see it. Some no name was spinning his scimitar in a circle like how a fan rotates and closed in on Ken who just casually punches him in the face between each rotation. There's more that I can share if you're interested. All of this is why I said that everyone named will either utterly destroy him or lose to him.
He's made a waterfall flow upwards with just his aura.
HSK (Hokuto Shinken) focuses on destroying the body from the inside out, so it's all about striking pressure points which can either make the body explode, shut down, or even heal others. He has moves that lets him control your body.
He also has a few projectile moves, tons of long ranged moves if you count Nanto Seiken but as I said earlier he won't use it under CIS rules or whatever it's called here where they fight in character.
To clarify earlier when I said he had low piercing durability. The instances where he's been pierced were with normal arrows but it didn't hurt him really. More like a needle prick. Once was when he was paralyzed and Raoh (I think) shot an arrow at a girl and Lin was begging him to save her so through willpower he broke free and was fast enough to intercept the arrow midflight and take it.
Here's a scene of that sword feat I was talking about.
In HNK 2 he fights against Kaioh, the master of Hokuto Ryuken and the technique Anryu Tenha which warps space around the opponent. At first it pretty much shut Ken down, but after mastering his MT he learned how to counter it easily.
After gauging his strength and getting tossed around he becomes a stone wall basically and easily tanks everything. That dragon breathing technique is shown in this clip.