also... with your thing about hard rockin' and stuff, I'd say that although they were consistently hard rocking, Led Zeppelin had some completely hard rocking stuff.
Black Dog is a perfect example of everything you mentioned - hard rockin', explosive, it's the best album opener I've heard.
"Communication Breakdown" is perhaps one of the best album-opening tracks of all. Considering when it came out, people must have thought their record players were exploding.
Angel Of Death isn't the best, but it is one of them. I have always liked the way Metallica open albums, Fight Fire With Fire, Battery, and Blackened are some of the best ever.