It seems popular to think that Load, Reload, and St. Anger are the worst Metallica albums. If that's the truth, they fell off after the Black Album. In this case, Death Magnetic is a comeback album, not the creative zenith before their worst albums because it happened after their worst albums. With that said, my vote would be And Justice For All... if we're speaking about their creative zenith. It's the most progressive musically. The Black Album is more representative of their sound at the zenith of their popularity.