There are so many good ones and they're hard to actually rank.
My personal favorites would be Buscemi's character in Reservoir Dogs, Harvey Keitel both in Reservoir Dogs and Pulp Fiction, and Michael Madsen in Reservoir Dogs. Clearly there are many other great ones though.
Fortunately for us, Tarrantino has tended to use great actors who have significantly enhanced the characters they are protraying.
My personal favorites would be Buscemi's character in Reservoir Dogs, Harvey Keitel both in Reservoir Dogs and Pulp Fiction, and Michael Madsen in Reservoir Dogs. Clearly there are many other great ones though.
Fortunately for us, Tarrantino has tended to use great actors who have significantly enhanced the characters they are protraying.