Musashi, without a doubt.
Conan has strength, but little skill.
Musashi did not kill Inigo's father, and so would easily defeat the revenge-driven Spaniard.
Yojimbo? I honestly don't know who that is.
Robin Hood was tops with a bow, not a sword.
Zorro is too wussy.
Connor McLeod has near-invincibility going for him, but if Musashi knows how to kill him, he can do it.
Leonardo would tell you, I'm sure, that he's a student of Musashi... not the master of him.