As unoriginal as this opinion may be, Led Zeppelin stands alone at the top. They are the Michael Jordan of Rock music. They had the technical chops, the longevity, the passion, the uncanny live performances, the chemistry, and the originality. All subsequent heavy metal bands were in some way influenced by Zeppelin. But Zeppelin wasn't just a heavy metal band that threw in a few cheesy ballads now and then. What I think made Zeppelin truly great was the diversity and range of the music they produced. They could play any style of music while still retaining their signature sound. And none of their music sounded trite, commercial, or manufactured. It all seemed to come from an authentic core. There is a "realness" to all Zeppelin songs. And like all truly great art, their music stands the test of time. One of the hardest things to do is to make music that doesn't sound dated. How do musicians know that what sounds good at the moment will still sound good decades later? But Led Zeppelin was somehow able to experiment with an array of sounds without creating music that seemed confined by the trappings of the era. There may be other bands and musicians that are just as technically proficient, but Zeppelin had all the intangibles and the combination of all the ingredients that create greatness.
Just my opinion.