It really doesn't matter which 3rd party you vote for, so vote for whichever one you abhor less or like more.

The main thing is that you're entering a vote that isn't being counted in the Dem or Rep column.

If you don't vote then it doesn't count against the Dems or Reps at all.

Vote whichever 3rd party is easiest on your conscience, and you'll be voting against both parties that can't responsibly work together at the moment.