I have two favorite classic art pieces which I have poster copies hanging in my house. Both of these pieces I fell in love with during a college art appreciation class.
The first one is Starry Night by Van Gogh. I have the poster (framed of course) hanging in our celestial themed bedroom. Just gives me a sense of peace.
The other is Scream by Edvard Munch. These picture just reminds me of mental illness and depression and such. It is how I truly felt (and still do feel) on my worst days of depression. Standing along just screaming.