Who would win in a Duel? Severus Snape or Sirius Black?

Snape most definitely. His role as a spy shows that he had incredible powers even at the age of 21. His Occlumency is strong enough to withstand Voldemort, who is considered the greatest Legilimens the world has ever seen. He is able to hide his true thoughts and he can do so without Voldemort suspecting a thing.