Well, maybe not the single best website regardless of topic or function, but the best site for environmental information with an emphasis on climate change.
And maybe the best sit ever in the past or future of humanity, but the best one I've found so far.
And it's hosted by a university! Yale, in fact (home of one of the best forestry/environmental schools in the USA).
It's called Yale Environment 360, and it's here.
Whether you're a sustainability administrator, a faculty member tasked with addressing sustainability issues (either by your department or by your students), or just an interested onlooker (like you can be an onlooker to a train wreck while you're riding the train), it's an excellent resource, constantly being updated. I'm going to recommend it to a number of the professors at Greenback. And to their students.