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.