What’s the least important principle of art?

That truly depends.

Explanation:

I don't feel as if any aren't important, but there are times where some principles aren't very much needed. Say you are painting a portrait of a friend indoors, nothing around them is happening. If they're still then there is no need to use movement in this piece.

i believe their role is to instruct certain sections on when they are to join and when they are to stop