First off, I can't believe that somebody actually voted for the bible of heavy metal "Master of Puppets".
As to the question at hand, anything since Load has been a load of shit, so although I could have picked Reload as the outright worst album, I went with the one that started the sucking process.
Although some people credit the black album as being their downfall, I disagree. Aside from tightening the songs (4-5 minute songs are more likely to get radio play than 8-9 minutes), I respected Metallica even more after this album, because I felt that they didn't significantly change their sound, but made it mainstream on their own terms...which is all the more reason why Load baffled me: they proved they could put out a metal album and have it be successful, why do an about face?
I think Garage Inc was a big mistake on their part. It allows even the new fans see how far they've fallen by comparing the good old days on disc two with the majority of the crap that they've become on disc one.