A resistor is one of the few devices in the universe that is 100% efficient, as all of the electrical energy supplied to it is converted to heat!
Or... 100% inefficient! :D Assuming you are not trying to build a heater, I'd say all of the energy dissipated by a resistor is wasted.
If your goal is to generate heat, a lot of things are 100% efficient. A light bulb generates light & heat and when the light hits an object and gets absorbed, the light is converted to heat. A speaker generates sound & heat and when the sound hits an object and is absorbed the sound is converted to heat.
In the winter when you are heating your house, none if this energy is wasted, since it means you don't have to use as much fuel heating your house. (But electrical heating is usually not the most economical.) If you are running an air conditioner in the summer all of that heat energy is wasted, plus it takes more energy from the air conditioner to remove that extra heat.