I think my favorite is still N.C. Wyeth, despite Rockwell's tremendous talent I think Wyeth had more breadth of imagination and a stronger sense of form.
Treasure Island
Kidnapped
Robin Hood
For sheer drama and overall uniform quality, you can't beat his illustrations for Treasure Island. But lots of others are great too -- that last one is a portrait of the carpetbaggers from the Pike County Ballads - an o.k. book redeemed by splendid illustrations.
J.M. Flagg
John Held
Charles Dana Gibson
Antonio Vargas