First off, there doesn't seem to be much benefit in taking more than what's already present in a cup or so of good green tea, though, so the exact content in varieties probably doesn't matter.
If you ever get bulk l-theanine powder (it's quite cheap), you'll recognize the taste. It's particularly prominent in sencha, gyokuro, and other Japanese green teas - that savory/salty/MSGish taste.
Anecdotally, l-theanine negates most of the jitteriness from caffeine for me. Coffee, pop, etc. make me edgy in a way that tea doesn't.
If you like scotch, try lapsang souchoug - it's a smoked Chinese black tea.
If you like sushi, try Japanese sencha or gyokuro - the same umami flavors in nori and kombu come through in the tea.
Etc. Etc.
If you were me, I would suggest a "malty" Assam (or an Irish breakfast blend) for most mornings, lapsang souchong for cold winter weekends, bundled up with crossword puzzles or programming texts, cheap-but-decent Assam as a base for making masala chai (with fresh cardamom and vanilla sugar), and good Sencha for the office. Citrusy green teas are great for summer pitchers of iced tea.
A good tea shop will be able to find matches for what you like, though. (Also, somebody at Tea Gschwendner (in Chicago) taught me that, after smelling lots of interesting teas, the best way to reset your sense of smell is to smell your clothes. FWIW.)
Anything you like, it is best to approach tea by taste, not by health benefits... start with experimentation -- whites, greens, oolongs, yellows, blacks, pu-erhs all have extremely different tastes.
I'm able to buy loose leaf in bulk from a local store so I can't recommend a particular online retailer. In person I would go by smell rather than brand, the aroma of good sencha is very distinct.
