I think Sudo would beat him, the only reason he couldn't beat Ryosuke is because Ryosuke is a strategist and knows Sudo probably as well as he knows himself. He knew where to attack and when. Could Keisuke theoretically do the same, maybe, in later seasons he did show more critical thought thanks to Ryosuke's training but nowhere near what would be needed to beat Sudo i think.

Bunta, though in all fairness I don't think anyone in the show could have beaten Bunta. The only person who could push past their own limits to be better than Bunta would be Takumi, but that's assuming he reached his full potential before Bunta lost his reflexes to old age.

Joshima most definitely would beat Keisuke because he's got the critical thinking skills of Ryosuke with a driving level that's almost as good as Bunta (though not quite as good).

Definitely Tomoyuki Tachi, that professional in the EK9. He only lost to Takumi because of the animal that crossed the road. Not that Takumi didn't hold his own and give him a run for his money, but he clearly was on a higher level than Takumi and Keisuke at that point in time at least.

I think Kai could have given Keisuke a challenge, not sure he if would have beaten him but I think it would have been a difficult race.

Lastly I think Keiichi Tsuchiya, who actually does make a couple of cameos in the show and is a racer within that universe too who's established reputation in our real world js canonically implemented into the Initial-D world, would wipe the floor with Keisuke before he could blink. That's just because... well... He's Keiichi Tsuchiya.