Much depends on what you wish to use your cover for.

I've used three different kinds of fabric; 100% cotton, Noah and Stormguard from Beverly Hills Motoring Accessories.

Stormguard from BHMA was a fabulous material, but it was bulkier than the other two materials, And if you had to remove it when wet it was cumbersome. However it worked very well in snow and ice.
Also, the pattern the way it was cut was very a very lose fit for the car, not a bad thing, but wind had an easy time of getting under the cover, even when tied down.

I ordered my 100% cotton from BHMA, I've had the cover for over SEVEN years. It also has a lose fit and over the years a hole has formed on the front pocket for the Mercedes emblem. The 100% cotton (in my opinion) is extraordinary in the way it keeps the car cool in the summer heat. The outside of the cover has bleached white formt he sun and the inside is still the orignal biege color - but I am not certain how well the 100% cotton inhibits UV light from the car - but it is my preferred cover for hot days

I have purchased three Noah covers for my Mercedes. One cover fit tight - but that may have to do with the mild body upgrades on my model year and Mercedes not upgrading the cover pattern. But the cover has lasted through snow and ice and hot summer sun. ( Be careful not to remove the Noah cover with ice or frozen precip on it, even peeling ice off of a Noah cover can rip the material ). I do use the cover in snow, but take extreme caution when removing snow and ice.
I like the Noah - and it is a paper like multi layer material - breathes well and only a few ounces of rain get through the material in a hard rain.
In the hot summer I use it almost daily on one of the cars, I've had to replace one of the Noah covers after three years due to the fabric ripping.

If your interested I can write more about the pros and cons of using a car cover in terms of the car finish, just let me know.