All electric heaters are very efficient at converting electricity to heat. Little is wasted (the fan in a fan heater uses very little) so the electricity you buy is effectively 100% converted. That doesn't mean they are the cheapest to run because you have to factor in the price you pay for the electricity. That's why electric storage rads work for some people - the conversion to energy process works the same but the electricity is bought at a lower price normal normal rate. Don't get conned by 'halogen' or 'ceramic' labels on heaters. They still work the same in terms of conversion efficiency.
Perhaps unsurprisingly, paraffin heaters use no electricity! i