Slayer is about as far on the "heaviness" scale as I like to go, but I do like them a lot. "Raining Blood" is a classic featuring one of the all-time great metal riffs. The sound of rain and those drums kicking in still gives me a chill, decades after I first heard it. In my opinion this is heavy metal at its most brutal and primitive (played live the bit from 2:10 to 2:38 results in an instant mosh pit; I've seen these guys in concert and the reaction is pavlovian). Yes, there are "heavier" bands, but Slayer still remains recognizable as music--a savage and scary brand of music, but one performed by talented musicians. Your mileage may vary, of course.
Turn it up, but not too loud, lest you frighten your household.