Arguably, oil-filled radiator heaters are the most reliable and economical one. Because the energy that is being produced goes to the oil rather than being discharged, and eventually getting lost in the whole room. Heating elements consume a lot of energy when it is turned on, but since there is heat sink which is created by the oil, less electricity is being used.
An electrical current is being used in the thermal oil which in turn heats the oil then allows it to be circulated all over the heater, just like water inside a radiator. It may take time to probably heat up the whole room, but once the heat is emitted, it will stay for long even if the portable heater is already turned off. This of course saves more energy because you don’t have to turn on the portable heater for long.