I like fantastic portrayals of people and scenes, and also enjoy surrealism and Pre-Raphaelite painters. Abstract "art" belongs on laundry detergent boxes and cubism gives me a headache.
The Pre-Raphaelites painters were prominent from the middle to the end of the 19th century, roughly corresponding with the Victorian Era in England. Their works are softly sensual and romantic. The subjects of their paintings are often mythological characters and archaic pastoral scenes.
These are some other artists whose work I enjoy.
Michael Whelan is another one of my favorites. You can find his work on many science fiction book covers, such as Arthur C. Clarke's "The Songs of Distant Earth", Robert Heinlein's "Friday" and Isaac Asimov's "Foundation" series. His compelling works set the mood for the tales beneath the beautiful covers.