Hulk would be scarier, but neither hero would be the ultimate in fear. That crown would be worn by a DC martian, like J'onn J'onnz driven insane. I really can't imagine much worse than a being that can read your mind, know your true and deepest fears and than literally use their power set to BECOME your worst fear. Superman and Hulk, no matter the level of power they reach, could never hope to match the terror someone with DC martian abilities could inspire.