I would say, Hercules, though Achilles is also pretty powerful. He does have invincibility, but I think Hercules would just pick up Achilles and repeatedly smash him into the ground and stab him until he hit his ankle, and Hercules found out about his weakness (Hulk smash him basically)
Also, I think Hercules is faster than Achilles, since he has the physical stature of a god, because of Hera's milk. He was able to hold his own against an enraged Apollo, and an enraged Apollo is no joke
And yes, I have taken Percy into consideration. I think Hercules would beat Percy unless Percy was fighting by an ocean. Hercules is just too fast and too strong, and I doubt Percy could hold his own against a fully enraged Apollo
Same thing for the other demigods. I think he could kill them all before they could react/think