Haven't seen the list, but I tend to disagree with any Black Sabbath song as a top-lister in the metal genre. They started a majority of the trends in contemporary metal, sure. But grass & leaves was the first material ever used to build a house, and it sure as hell wouldn't rate #1. Black Sabbath's Iron Man is as a metal song simply dull, plodding, and uninspired. It's a simple melody that would have easily been a written-off folk song had it been produced acoustically and sung by a hippie.
Personally, picking a "best metal song of all time" would involve popularity, production quality, and overall impact.
After thinking about it for a moment while taking a break here at work, I've come up with Metallica - Sanitarium. Not my favorite metal song in the world, and to be fair I don't know anyone out there who would call it their favorite metal song. But of all the metalheads I know (and I know way more than I'd ever care to try to count), I have to say at least 9 out of 10 have always agreed in this particular conversation (that I've also had many times) that they spent at least a month of their teen years with that song blasting over and over in their walkman while holed up in their rooms. No other song can lay a claim like that, methinks.
