John Buscema is one of my all-time favorites. From his famous run on Conan to his one-shot on Howard the Duck (issue #3), his art never failed to draw me into the story.Some others I loved in the comics field: Michael Golden, Frank Brunner, Gene Colan, and Steve Mellor (the Erol Otus of the comics field, IMO).
Silver age art is old school? urgh... old school so confusing...
I'll add Alex Toth, Richard Corben, Mike Kaluta and Jack Kirby to the list.
Alfredo Alcala rocks.
Not enough Alfredo Alcala out there in the blogosphere, sadly.