As far as Metallica goes, I'm still partial to "One". Sorry, I know it got beaten to death on MTV, but it just drips with raw power. And as far as pure evil goes, I'll take White Zombie any day. "Real Solution #9". I always get the drivethrough at McDonalds bangin' with that one.
But as we all know, "the greatest song ever" will be just as subjective as your thread about the greatest drummer and guitar player. So, that being said, I won't vote for the greatest song. But I will take a little trip back in time and give you some of my classic favorites.
"Fool In The Rain"- Led Zepplin
"Comfortably Numb"- Pink Floyd
"Join Together"- The Who
Perhaps a few for artistic merit:
"Xanadu"- Rush
"The Wind Cries Mary"- Jimi Hendrix
And for anyone who has seen U2 live, there's nothing like hearing tens of thousands of people singing "40" and they slowly file out of the stadium.