There is no "best click through rate" define by google. It depends on your niche - 1% might be good for certain sites, other niche sites might be 20%. I've had sites consistently do 30% - but you can have low traffic channels with a 300% CTR (1 visitor, 3 clicks) on certain days.
AdSense advertisers and publishers serve a very dynamic, diverse market. What may be a high CTR in one industry or vertical may be considered low in another. So, it's difficult to determine an "best" CTR for everyone.
CTR depends on the ad types that you are using plus the placement of ads plus the Niche. There is no specific best CTR for all sites. One site's best CTR may be bad CTR for another site. For example: forums have low CTR so we cannot compare it to a blog.
Best CTR is anywhere between 5 to 10 percent. And it always depends on the type site and traffic you would be getting.