While Fade to Black and Sanitarium have great solos, for balls ass metal solos I would have to say that Frayed Ends of Sanity, Dyers Eve, and Struggle Within are at the very top.
And though he rarely solos, and his solos are nothing special, and the thread is about solos, without question, the undisputed king of down picking is James Hetfield.
It's the intro and bridge solos in Sanitarium and Fade to Black that really get me. They're simple, but they fit so well. They are the first two solos I ever learned to play.
Hammett studied under Satriani for a while. I'd love to hear those guys do a project together.