"War Pigs" is the best protest song in the history of forever, especially the parts starting at 3:20 and 6:07.
The opening riff of "War Pigs" is easily one of the most awesomely ominous E-chords ever strummed. It's not hard to imagine the apocalypse being ushered in by that note. It sounds just like THE BELL OF DOOM.
Faith No More's cover is pretty damn good, too. As for Sabbath originals, there's "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.
"Children of the Grave" has such a badass opening riff that it was used in the video games Brütal Legend and Guitar Hero: Warriors of Rock.
"Megalomania". The studio version is awesome enough, but this 1975 live version takes it to another level, especially after the 3:50 mark. Over the top of an absolute dinosaur of a riff, Ozzy jettisons actual singing for anguished shrieks, and sounds utterly possessed.
"The Wizard" is just bluesy enough to rock out, and just heavy enough to fist pump to. And it shows off Ozzy's harmonica abilities.