I heard a stat at a conference recently that 15% of the world's best-ranking websites are WordPress sites.
Imaginary garbage! It has NOTHING to do with a site being driven by WordPress. Anybody can prove ANYTHING with random statistics and/or percentages created out of thin air.
First of all Search enginge do NOT rank 'sites', they rank URLs.
Secondly; it depends TOTALLY on what anyone searches for.
The things I predominantly research using search engines, brings up StackOverflow URLs 75% of the time so from that I can deduce that more than 50% of the 'best ranking sites' are ccWiki, (which is what 'powers' the site at StackOverflow).
Wordpress powered sites appear in results more often, simply because it is THE most commonly used CMS, NOT because it has some magical power for "SEO" which is what that 'statistic' is meant to have you believe.
The old advice of "Beware of Greeks bearing gifts" should be rewritten to be "Beware of SEO 'experts' quoting statistics"