All electric heaters are about 100% efficient so produce 1kWh of heat for every kWh of electricity. There aren't any that are more efficient but they do differ in how they deliver the heat. It's your choice of what you want. A fan heater, infra red heater, convection heater or an electric radiator.

@M.isterW would an electric radiator tend be oil filled? We have one and it’s big, heavy, slow to hear up, but keeps the room warmer for longer. A fan heater is a way to get a short blast of heat that goes quickly. That’s my experience at least. @Beaky22 what sort of use case are you looking for?

@M.isterW with the 100% efficiency in mind, it’s safe to assume that some heaters use more kWh than others - I imagine fan heaters to be the most costly of these if left on for 30 minutes compared to others but this is guess work. I’m hoping others may have something more concreate.  In the meantime, OVO have created a guide here:

What are the best energy-efficient heaters for your home?

There's a great podcast on this subject, available on BBC Sounds. It's part of the "Sliced Bread" series by Greg Foot and I can recommend listening to it to find out more about the different types of electric room heater.

https://www.bbc.co.uk/sounds/series/p07012sf