colour pencils
Pencils give the artist the greatest amount of control, and for that reason, I love to periodically go back to pencils to hone my basic drawing and shading skills. It is the medium easiest to use and simplest to clean up after. 

Most importantly, there is a special kind of joy in holding a good quality color pencil in your hand. The feel of the Faber Castell polychromos (my brand of choice) pencil touching paper is luxurious. Colored pencils, in my opinion are the most underrated mediums.