What is the best film festival that requires that my film has not been shown at any other festival?

What is the best film festival in the world that requires that my film has not been shown at any other film festival before being shown at it. So like it requires that my films first screening is at there festival. I ask this because I don't want to pass up that chance before I start showing it to others and lose that opportunity.|||This site has alot http://www.britfilms.com/festivals/|||I think Halloween