Their self-titled song. A pure and prime example of "Diabolus in musica." The song was inspired by a nightmare of "Geezer" Butler's. After reading a book about the occult, he fell asleep and woke up a bit later to see a silent creature with a black face staring back at him. And black means pure black. No eyes, mouth, anything - onlyblack.
When it first came out, this song was so scary that the first time they played it, most of the audience ran out of the venue screaming. (The ones that stayed yelled for it to be played again.)
Another common interpretation of the song is that it's about a dabbler in the black arts who inadvertantly summons Satan himself. The unlucky summoner is then dragged kicking and screaming down to Hell by the Prince of Darkness.
Listen to the demo version of this song, Ozzy sounds more like a victim of the demonic summoning (even though he was the one caused it, well, technically Geezer... but still) and the extra verse can frighten people.
"Hand of Doom" from Paranoid has frighteningly explicit lyrics about drug addiction, especially in the last part of the song, coupled with occasional, brief bursts of guitar feedback that sounds like a drill entering your skull.
"War Pigs", also from Paranoid. The first and third part of the apocalyptic anti-war lyrics are accompanied with eerie silence save for the regular guitar hits.
"Electric Funeral", again from the above mentioned album, is pretty damn scary for a song about nuclear war, with its nightmarish description of the radioactive fallout and the chugging pace of the riff, which feels like some sort of lumbering monster or something.
"Electric Funeral" also contains an unusually terrifying example of the band showing their work. In the third verse, the reference to "terrifying rain" calls to mind the rain storms that followed the atomic bombings of Hiroshima and Nagasaki. Thick, black rain, filled with the ashes of burned buildings and people sucked into the mushroom cloud fell in the aftermath of the blast. Many victims drank the water to, as the song states, "ease the burning pain", not knowing that it was highly radioactive.
The cover of Black Sabbath. A figure in black standing in front of a dilapidated house in a wooded area, an image that hits on the same scary imagery The Blair Witch Project covered thirty years later. And who is that? Is it Death? Satan? A witch? Or just a creepy hermit? And what is it staring at?
Much of the final short, "The Drop of Water", but the ghoulish Uncanny Valley face of the dead medium stands out.