The great H.R. Giger definitely springs to mind. I love the fact that most of his works are devoid of colour yet are still incredibly vivid and sometimes strangely erotic:
I would love to go to the Giger museum some time. His imagination is astonishing.
Monet is another favourite. Cliched I know but I love his work for the exact opposite reasons to liking Giger. His pictures are bright, colourful and evocative: