Car Cover material

What's considered the best material for a car cover?

I know that breath-ability is an issue, so is cotton best, or something synthetic like the material the hardtop cover is made of?

[Quickdraw]

I have the Ford Thunderbird cover, it's a Noah with wing logo on the front. I use it indoors so anything would work fine but this fits like a glove. Retail on this is about $225 but can be found in the $150 - $175 range.

Heres a comparison chart:
http://markii.com/covers/fabcomp.htm