As a slavish Cole fanboy I would love to vote him #1 but he did very little interior work and I can't vote someone who mainly did covers as the greatest artist overall. From an art perspective I find Cole's work with color and his ability to be eye catching to be really artistically appealing, though I find his figure work to be somewhat lacking. His surreal and Sci fi covers are most appealing to me. With that said, he did produce some real mediocre work, which is bound to happen when you draw something like 1500 covers.
Baker was second only to Frazetta in his ability to capture a moment, except Baker had a unique way of pulling the viewer into the scene to evoke a sense of nostalgia that I don't think any other artist, of any age, can touch. Unfortunately he also didn't produce much interior work and what he did doesn't really stand out.