It's hard to say what is the most powerful metal song in my opinion. I could say Marduk's entire "Panzer Divison Marduk" album, as it's non-stop agression. That doesn't mean that any of them are my favovorite metal songs though. My favorite "metal" song is "Nightrain" by Guns N' Roses, but I don't know if it's that powerful. After the solo it is though. "Black no. 1" by Type O Negative comes in after that, but that's not powerful at all.
But overall I would say that Marduk record, as it's the evilest, hardest, fastest, most brutal album ever released!