Who else would come in at #1? Rackham remains the single most amazing artist and illustrator of his day, and though he isn't as widely recognized as he used to be, he remains one of the most celebrated and highly collectible artists in recent years. Whether creating hauntingly melancholic images as in Undine or breathing new life into ancient mythological figures such as Pandora and Thor, Rackham proved to be the solitary master of the Golden Age of Illustration.
Froud is one of my favorite illustrators and conceptual designers. His artwork, much like Rackham's, is both ethereal, beautiful, and creepy. The work that he did for The Dark Crystal is so unique and really set the tone for the film perfectly.
Doré's work on everything from historical volumes to The Bible to Dante's Divine Comedy and Milton's Paradise Lost is superb. I especially love his Gothic visions of angels and demons, and his classic Satan Contemplates a Serpent is a personal favorite of mine. So spooky.
Dulac's illustrations for numerous children's books and fairy tales are superbly done. I particularly like his depiction of exotic Middle Eastern and Asian cultures. See Princess Badoura for an example.
Wyeth is certainly one of the most celebrated illustrators, particularly for his work on classics such as The Boy's King Arthur and Treasure Island are so beautifully created that they almost overshadow the books in their appeal.
Frazetta has proved to be the king of "pulp adventure" and "sword & sorcery" art. Though some have attempted to dismiss his work because of the genres which he worked in, there's no denying his masterful talent when it comes right down to it. Whether illustrating Vampirella or creating amazing images for the paperback editions of Conan, Frazetta was and is incredible!