Depends what you want to use it for. Is the car garaged and you just want a dust cover, or is it permantly parked outside in the weather?
Is it likely to be on the car occasionally or permanently and for long periods of time?
If it is inside then a basic nylon cover is all you need. If it is kept outside that is an entirely different proposition.
"Fully Waterproof" covers are usually not the answer because they do not allow 'breathing' and cant be left on for days or weeks at a time without potentially damaging the paint.
In my experience the best material for a car cover is TYVEK. It is 99% water resistant, ver light weight, very strong and most importantly, breathes. You can get this material in KENCO and also AUTOTECNICA brands.
Equally important is to get the size absolutely right. If it isnt a tight fit it will move around with wind which will end up scuffing your paint surface.
There you go, car cover lesson 101