Hmm I was rather surprised at the results. I was sure most would go for MoP, which is imo their best song ever, but I guess I was wrong...
Anyway, I went for Master of Puppets. The riff, singing, (incredible) solo and kickass combination and timing all made it a classic, and it deserves to be so. It stands to remind us that Metallica were once much much more than money grabbing whores