Ranked by useful heat delivered per unit of energy put in. A heat pump moves existing heat rather than generating it, so it delivers several units of heat per unit of electricity and sits well above anything burning fuel. Among the combustion options, a mass heater extracts more from the same wood than a plain stove by storing a short, hot burn instead of sending it up the flue.
Solutions
Wood stove / mass heater: Biomass-fuelled backup heat; mass heaters store and slow-release heat from a short, hot burn.
Heat pump (air- or ground-source): Electric backup heating; efficient but adds a powered, less-repairable component (cross-ref power dependency).
Rocket mass heater: High-efficiency biomass combustion chamber feeding a large thermal-mass bench that slow-releases heat over many hours from a short, hot burn.