Obviously wikipedia.org. Nothing better has been invented in 20-some years. I honestly feel relief when I research some topic and it has Wikipedia page on it, so I don't need to dig through some sh**y blogposts

Edit: I mean: it's one of the very few websites that actually provides sources of information. It's insane it's that rare these days. And most important facts are just placed in visible and easy to spot place, no need to dig through author's life story, thanks for subscribers etc. before getting to the point.