Henri Matisse - 'The Red Studio'. The graphic nature of this painting really appeals to me, the play of space and use of line that just gives enough visual hint that you're not just looking at a flat surface. And the paintings, wine glass etc brings in a sense of life and energy to what just seems a still life.
Lichtenstein. Hard to pin down an absolute overall favorite but this is definitely near the top of my list. I just love his graphic style and way in which he composed his images.