I like to think a lot of modern art is just one huge practical joke...there's a lot of "a polar bear frolicing in the snow" sort of BS going on with a lot of the paintings in my Art History textbook. Perhaps the funniest example of this (and if anyone knows what I'm talking about, please provide a link so everyone else can see) is a "painting" I saw in Washington, DC, which is basically a series of single lines, going down in a sort of south-east direction, all different colors, with nothing in the middle of the canvas....is that what passes for art these days? 
To get back to your question, I have a grudging respect for Warhol (I suspect he may have been the biggest practical joker of them all, what with the soup cans), as well as Jasper Johns and Roy Lichtenstein. As for the classics, Rembrandt, Van Gogh, and Paul Klee are pretty good.
(and then there's DaVinci, Michelangelo, et al, but I won't bore you with that)