Fountain pen with spilled ink.
This is one of my favorites, mainly because it turned out how I wanted it to. So often I start with an idea in mind and I can’t draw well enough to get it out how I want. Incidentally, this has been one of my best sellers on Cafepress (which means it has sold like 6 or something).
I really do enjoy writing with India ink and a fountain pen. It’s fun to see your line width vary so much depending how you hold, angle and press the pen.