MDN always has the most relevant and easy to read documentation for me, but it's a shame it doesn't get ranked more highly in search engines. Whenever I forget the syntax for a JS or CSS property, I'll always type it into Google and append MDN to make sure I get what I want quickly.
It is a shame. I'm constantly astounded and frustrated at how effective w3school's SEO is. Their content is not terrible by any means but it's simply not nearly as comprehensive or authoritative as MDN and others. And because of the name, quite often they even outrank w3.org when I'm trying to find the actual specifications themselves!