"Best" is very subjective. Best looking? Protection? Ease of use? durability? cat proof? light fastness? etc..
The best outdoor car cover in my opinion is a cheap tyvek one I buy every 6 months. The spun poly is scratch free, installs in about one minute, take off and rolls up nicely in under 30 seconds, water proof, breathable, light weight.
Of course it doesn't fit like a glove like the $500 factory outdoor cover (which I have and it's for sale if you want it).. And for $70 it's hard to match. Just they don't last very long.