IN YOUR OPINION: "Most Memorable Metal Riff"

Which song do you think has the most memorable metal riff? 

IN YOUR OPINION

IYO: Which song has the most memorable metal riff? "Walk" by Pantera "Paranoid" by Black Sabbath "Raining Blood" by Slayer "For Whom the Bell Tolls" by Metallica "The Philosopher" by Death "The Optimist" by Skinless "Spheres of Madness" by Decapitated