Ideally you want a breathable one. So that it protects the paint and the cover will not adhere to the paintwork and pull colour off.
Although the downfall to that is that they let water through as it is a permeable membrane.
My personal choice would be to get a permeable membrane, then get some waterproof over the top of it. I.E. Breathable car cover tailored to the car, then a large sheet of tarpaulin over the top of that.
You should expect to pay ~£40 for the cover, then ~£10 for the tarpaulin. Oh and some bungee cords can help to pin corners down, and go underneath the car so as not to catch or scratch bodywork.