I think I like all those six first as much. Sabotage is also really great! Symptom of the Universe, Megalomania, Supertzar, Am I going Insane, really great classic Sabbath tunes! Hole in the sky has of course great riff, but I think in that song band started to go to the little bit boring direction. Of course there are some not so great songs also in the earlier albums.
But if I have to say my favourite of those I think itīs also Vol.4. Maybe just because it was my first Sabbath and also my first rock album I remember to listen.
About my parents, they introduced me only this:
https://www.youtube.com/watch?v=WE5SrkzxYdY