I don't think any bands are more universal than Sabbath and Maiden, and to a lesser degree Judas Priest and early Metallica. Anything with extreme vocals will immediately alienate a large number of metalheads. On a statistical basis, there are a lot more people interested in trad metal vs. extreme metal, but metal forums skew towards extreme metal. Plus, people's first experiences with metal more likely than not involves one of the bands I previously mentioned.
I mean, no one can hear War Pigs and not be totally engrossed. That is probably the best expression of metal, at least 10 years before any other band came close to that.