I don't think there is any one source for the best tea of all genres. For example, my favorite lapsang souchong comes from Grace Rare Teas, my favorite dragon well and most of my favorite black teas come from specialteas.com, and my favorite favorite flavored green tea is the pineapple ginger green tea from Republic of Tea.
In general though I recommend specialteas.com, I went through a major tea phase five years ago and they quickly became my favorite web shop. Their organic dragon well is really good, as well as their mount everest blend and their vanilla sunday blend. I also really like mate de coca, which you can now get from Amazon.
Well, I get some good teas from local Korean and Vietnamese grocery stores, but Adagio is a bit more accessible for most people here. :)
Roasted barley tea* (http://en.wikipedia.org/wiki/Mugi-cha) can be pretty good, too, both hot and iced. Korean markets often have big sacks of roasted barley for cheap. The roasted corn tea is also good.
* Not really a tea, it's...roasted barley. Surprise surprise.
Yerba mate is also good, but deserves a completely separate top-level post.
If you like mate then stop by The Mate Factor if you're ever in Ithaca. It's run by a cult whose religion somehow requires them to make the best mate in the world. You have to be part of the religion to work there, so I was seriously thinking about converting for a couple months just to learn how to make some of their drinks and food items.
It's worth pointing out that their website is very well done, especially the reordering process. Once you establish some favorites, it's a breeze to access previous orders and replenish the cupboard.
In general though I recommend specialteas.com, I went through a major tea phase five years ago and they quickly became my favorite web shop. Their organic dragon well is really good, as well as their mount everest blend and their vanilla sunday blend. I also really like mate de coca, which you can now get from Amazon.