Pick one! If you picked other, tell us what it is.
Exactly why/what/who would ever make it necessary for me to choose only one medium? I can't even conceive of it happening...
To answer your question anyway, it would have to be acrylic. It's the only medium that allows me to "draw" (due to fast drying), use color as either glazes or opaques, on any surface, make some form of prints, and allow me to create sculptural forms (only medium that can be used on both solid and flexible sculpted forms, to create three-dimensional forms using it as both adhesive and color/texture). There is no second place...
Sumi? isn't that ink and watercolours?
For me, anything liquid that is manipulated with a brush.
Hard one, but had to go with good ol' graphite.
Digital because it can imitate the rest, its the fastest, cleanest, easiest, manipulative-st, I've never heard of a pen that can re-size warp erase paint blur colour balance your paintings for you, well except wacom's.
What else can digital do?