"Best" insulator.............hmmmmmm. Heat is lost through Convection, Conduction, or Radiation. An "ideal" metalized thermos bottle minimizes all (3) losses. My vote for best insulator.
My guess is yes, bubbles help keep the "heat" in. Blanket on top the pool, is very good at keeping heat in.
All things equal, does a gallon of milk thaw at the the same rate it froze, or take longer to thaw?