These are not in order but they are:

1. chorus riff on master of puppets (metallica)
2. intro riff onHell awaits PURE METAL (slayer)
3. verse riff on battery (metallica)
4. intro riff on holy wars (megadeth)
5. chorus riff on I'm the law (anthrax) I guess its two riffs but i like em both the same


Honorable mention:

1.March of the S.O.D. You might remember it as the intro theme to headbangers ball The heaviest thing I ever heard back in 87 (S.O.D.)
2. intro riff to war ensemble (slayer)
3. riff on one right before the solo, arguably the illest metal solo ever (metallica)