I would have to say that currently the most valuable sites are those like k5 and /. - why?
Because they promote Free Speech - honest, free discussions, about nearly anything (k5 more so than /. - only because /. caters to a slightly different crowd). I have seen other sites running both Scoop and / that cater to widely varied interests, but they all form communities.
These sites are the "town squares" of the internet, a place where people can mingle in a virtual realm, see viewpoints from all sides (if they wish), and respond in kind. If they don't like what someone is saying, they can either moderate, or come up with a better argument: "The cure for bad speech is more speech" (or something like that...
The sites that use a form of moderation are the best, as they allow others to tell the cranks from those who can argue effectively, which makes for a higher quality discussion site. Many "normal" news sites could take a pointer from this in regards to thier "talk back" discussion groups which crop up on articles - generally those who really have something valuable to say are drowned out by trolls, instead of rising to the top where their opinions can be heard.
These sites are creating a "quite" revolution of sorts within the internet community - some are on the verge of being a real political force, which can affect the real world (indeed, politians are idiots not to adopt similar type sites for themselves).
What other sites would I rate high?
I would have to say formost Google - as a generla search engine it wins hands down. However, I would also have to rate sites dabbling in the documentation of the so-called "invisible web" as being invaluable - oftentimes these sites are the only link between the general internet, and isolated "islands" of data that exist out there.
I would also have to rate some .mil sites as being important - they allow the public (when the public finds them, that is) to see a side of life not generally known on the outside. Interesting is the artwork at dia.mil...
Textfile sites are valuable in that they preserve a history of online discussion, of a sort. Many preserve articles from the really old BBS days, that are still fascinating reads, even today.
There are a few outstanding commerce sites - I rate Pricewatch as being the most useful and valuable - allowing citizens (ne, "consumers" - I HATE that word) to get the best bang for thier buck...