Well, Dan, you did say "the least energy efficient ". Electric heat applied at the point of use, like a baseboard heater, is indeed 100% energy efficient, all the time. ALL of the electricity used is converted to heat.
That is quite different than whether electric resistance heat is "cost efficient". In order to determine that, you have to evaluate the cost of all alternatives, like natural gas, firewood, propane, coal, etc.
Even a heat pump may not be as efficient as baseboard heat in extreme cold. The resistance elements in a typical heat pump (used when its so cold the pump is inefficient) are not located in the living area, so some heat is wasted.
It all "depends".