I started to play Heavy Metal a year ago (I was playing Hard Rock and Rock'n'Roll before), and I don't know much Heavy Metal riffs, but I would like to investigate them. I hope, that this tread will be interesting for others too. Keep rockin'!

Anthrax - Armed and Dangerous chorus
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7---------10------------|------------5--5-10--8--
7---------10------------|------------5--5-10--8--
5----------8------------|8555558555553--3--8--6-- repeat

1.....2.....3.....4.....|1.....2.....3.....4

I really like using the Cowboys From Hell (Pantera) opening riff as a speed exercise. I found it's easier to play though when you start with an upstroke.
You repeat the main part three times and using the second ending the fourth time through.
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-|x
-|x
-|--------------------------------------------------------12-
-|----------------12-----12-----13-----12------13/14-----
-|-12-15-12-15-----15-----15-----15------15------------

____________________________|13-12--------------------
___________________________|--------15-12~----------