I'm a giant fan of their Spiced Green and Gunpowder green varieties.
Incidentally, if you're anywhere near Chicago they have a retail store in Naperville where they'll brew up any of their teas for you to try. It's fun.
It has 31 single-serving sample teas, and a teacup with a strainer. The quality is reasonable, and the variety is spectacular. After finishing it, you'll know which teas you enjoy. I took notes to remind myself which teas I liked and didn't like, but it was worth it. Plus, I still use the teacup and strainer!
If you've got a good local tea shop, definitely give them a try though. They should be able to steer you towards varieties you'll really like.
I particularly like lapsang souchong (smoked Chinese black tea), sencha (Japanese green), and Assam (Indian black).
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.
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.
My favorite tea there doesn't get a high rating but I've had a pound of it in the last year and still love it. Green Anji: http://www.adagio.com/green/green_anji.html?SID=531a74998702...
Their masala chai and wuyi ensemble are also both really good.