I like Francis Bacon a lot, who someone else already mentioned. Also...
I'm a pretty big fan of Chuck Close. Unfortunately looking at a JPG online really doesn't do his work any justice. His paintings are huge and much more impressive in person. Anyway, here's one example:
I also love Joseph Stella. One of the neat things about him was his style constantly changed so his output was extremely diverse. Here's my favorite painting by him,
Brooklyn Bridge:
Goya, was also pretty amazing and I frequently find his paintings sort of frightening: