What resistor type is best for use as a heater?
All resistors have the same efficiency, they convert the dissipated power to heat. The answer depends on whether you want to heat air or a surface. A resistor that is mounted to the chassis with screws will transfer the heat to a surface more efficiently. A resistor with poor contact to the surface, will mostly heat the air.

Chassis mount resistor:


https://www.digikey.com/product-detail/en/stackpole-electronics-inc/KAL25FB1R00/KAL25FB1R00-ND/1646178