I'm a fan of Zeppelin, along with hundreds of other groups/musicians. "The greatest rock band of all time"? That's a subjective call and I can't answer. I was once asked, by a person who clearly considered himself to be uber knowledgeable on popular music, what my favorite group was. My answer was that I couldn't single out a favorite, and that I had hundreds of favorites. His response: That I obviously didn't know anything about music. ??????
I've got all of their albums. But, their first four are their best, IMO. ... OTOH, they never had a BAD album.