I picked Into the Void because that's the one that is the most repeatable for me. I play it all the fucking time and it is a ringtone on my phone for a few people.
That said, picking an actual favorite song from Black Sabbath is next to fucking impossible. I mean, the amount of good songs they have is beyond fucking ridiculous.
I am one of the lucky few who got to see Ozzy reuniting with Black Sabbath. Atlanta. 2001. Ozzfest. 2 stages. Music started at 10 am. It was an all day event. When they shut down the 2nd stage and got to the main event, the final 5 bands to go on were Primus, Slayer, Deftones, Rob Zombie and Black Sabbath.
Black Sabbath played a 3 hour set that ran past 1 am. They played 3 encores too. When they got to the final encore, about 15 minutes had passed since the 2nd. Half of the crowd had cleared out. Pretty much everyone thought the concert was over. And then this music hit...
That is quite possibly the hardest I have experienced in my entire life.