An type of electric heater is going to have an efficiency of at least 99%. Electric heating is efficient in terms of energy usage, but it tends to be the most expensive to operate.

You might have a hard time finding a small 120 V electric heater with a built in timer and thermostat display. Heaters with those features tend to be larger and operate on 240 V. Another option for a bathroom is a ceiling heater or an exhaust fan with an integrated heater.

Electric resistance heat has an efficiency of 100% because that's what its efficiency is defined to be. It's only relevant when comparing resistance heat to other electrical sources, like a heat pump, that might by comparison be 300% efficient because the different technology it uses to produce heat from electricity uses less electricity to do so. Or, you could say, produces more heat in BTUs for the same input. Like a car getting better mileage than a truck.

The absolute number of the "efficiency" factor is meaningless.

Energy efficiency is defined by useful power output divided by total energy input. You can get very close to 100% efficiency with electric resistance heating. There is some loss in the wiring between the circuit breaker panel and the electric heater. This is due to the wires dissipating some heat and the voltage drop across the wiring.

Most time people will use the term efficient when they really mean economical. Electric resistance heating has the highest energy efficiency, but in most cases it is the most expensive. On the other hand you could have a coal fired boiler which only operates a 50% efficiency, but if the price of coal is low it may be the cheapest way to heat your house.

As a completely different option: Space heaters have come a long way. I bought the Heat Storm "phoenix" and it heats up a 12x18 room easily. Can be wall mounted.

"Energy efficiency is defined by useful power output divided by total energy input. You can get very close to 100% efficiency with electric resistance heating."

Consider the words thoughtfully. A resistance heater converts the all the energy in the electricity to heat but does so INEFFICIENTLY.

-Natural gas is a major energy source to produce electricity in the US. If the natural gas used to produce the electricity were instead used in a home furnace to produce heat, a given quantity of natural gas used in the home furnace would produce considerably more heat (BTUs) than if used in a power plant to produce electricity that ultimately is used to heat a home with a resistance unit. Because home furnaces and boilers are more efficient, extracting and providing more of the heat content from the energy source than power plants and electrical distribution systems do.

-If the same amount of electricity as in the resistance heater is supplied were used instead for a heat pump, several multiples more heat (BTUs) output would be produced. Because heat pumps operate more efficiently to produce more heat from electricity than resistance heaters do.

So of these three choices to use energy to produce heat, electrical resistance heating is the LEAST efficient user of energy. Use the words however you want but don't mislead yourself or others. As I said before, the 100% number is a definition that is useful when comparing output to other sources but essentially meaningless as an absolute number.

What about electric baseboard heater with its own thermostat?