My best would be ~5% but that's different for every site.. Some sites make 10-15%, others can't get 1%, so - no universal answer to that one
I have sites with identical layouts but in different niches that average considerably different CTR. I think some niches of Adwords advertisers write better ads imo, regardless of any other factors.
It is hard to judge as different people get different traffic which will have an affect on the impressions and the clickthrough's so someone with 8% CTR could be excellent whereas for someone else could be rubbish.
i think the average CTR on a website with average content quality is around 5% ( im talking about the older doamins)
There is no "the best adsense CTR" but anything in double digit is considered good but watch out for the eCPM that ultimately matters more in earning...