Are we just talking about Classic Rock bands? Or classic as in 25+ years old regadless of the genre?
Because if it's the latter, I'd say Destruction and Exodus, and Slayer to some extent. Thrash Metal is insanely repetitive most of the times and it tends to repeat riffs every now and then. Oh, look, it's that riff that's similar with the one in the previous album. Wow, those are anti-religion lyrics. And here comes the dissonant solo by Kerry King. Oh, the next song isn't fast but it tries to be heavy, I see. Seriously though, Slayer has some great songs, but most of it is really stale.
I also don't know how to get into Early Pink Floyd (Pre-Meddle). It's not that I dislike it, it's just that... Well, I don't find them as good as their mid-later works.