War Pigs is the best protest song in the history of forever, especially the parts starting at 3:20 and 6:07.
Faith No More's cover is pretty damn good, too. As for Sabbath originals, I'm disappointed no one's mentioned "Die Young" or "Into the Void".
"Lost Forever", "When Death Calls" (With guest apperance from Brian May!), "Call of the Wild", "The Law Maker", "Jerusalem" etc. Seriously, the albums with Tony Martin deserve more love.
"Symptom of the Universe" is another biggie. It's a pile-driving number that helped inspire the thrash movement, but what really makes it awesome is that after completing the impressive main song, they float down with an effective acoustic coda.
If Children of the Grave doesn't have you headbanging by the first power-chord, you may not have a pulse. The opening instrumental Embryo should also get a mention for being so twisted and evil sounding.
The Wizard just blusey enough to rock out, and just heavy enough to fist pump to. And it shows off Ozzy's harmonica abilities.