my choice is not a popular one but i really love this song a lot, Nazareth - Broken Down Angel (1973) also I think the 1970's was the best decade of heavy metal.
'Hallowed be thy name' by Iron Maiden comes to mind if with heavy metal you mean only the classic / traditional heavy metal... If not (if you mean metal music in general)... maybe Satyricon's 'Mother North'. You ain't lived til you heard it.
Too many to choose from... Diamond Head - Am I evil, It's electric, Helpless or Deep Purple - Smoke on the water or Iron Maiden - The trooper, run to the hills, hallowed be thy name, 2 minutes to midnight and so on. Wrathchild is a very good one too. Metallica - Hit the lights, Whiplash, Ride the lightning, For whom the bell tolls, Master of Puppets Judas Priest - Painkiller Dio - Holy diver, Don't talk to strangers I think I have to go with Deep Purple - Smoke on the water or Judas Priest - Painkiller; these are some ultimate heavy metal songs in my opinion. But Am I evil is a very great one too..
Hmm...war pigs or sweet leaf. I like Sabbath's stuff, actually, but I like Gov't Mules covers of the two songs moreso.
The best heavy metal song, like wow... That's one of the hardest questions ever Ummm, I actually have no clue wich one I like best. They're all so different from each other and each one touches me in an other way, it also depends on my mood I think I'm gonna have to pass, I like music waaay too much to choose