Crazy Train.
I'd have to say N.i.B. - those lyrics by Geezer are both scary and powerful. Outstanding.
This technically is not a Black Sabbath song since GB didn't write it.
Paranoid because of the riff.
War Pigs
Just always loved that one.
When I was a wild child it was a constant of our group.
Vol. 4 was my first Sabbath LP, wayy back in '87 or so, as such it's probably my favorite album and I like all the songs about equally.