Edvard Munch's 'The Scream' has always been a favorite. The colors grab your attention, and though the style is simplistic and stylized, the emotion of the piece comes through. There is feeling and mystery here...are the people in the background just passersby, outside of the main figure's world of pain, or is there a more sinister intent ? And, honestly, another reason I like this painting is some days, I feel like this guy.
On the opposite end of the emotional spectrum we have Claude Monet. His expressionistic paintings have a serene , almost dreamy feel to them. To me, his works have an almost calming effect. His work has always been my favorite of the Impressionists.