Boy, you ran into just the right guy to answer this question... I tried several brands of Green Tea, found that most of them make a tea that is far too mild for my tastes. I wanted something strong tasting, something that tastes like TEA and has that caffeine kick that I need in the morning.
My favorite is a brand I find at Trader Joes, a local novelty grocery store. It is Salada. They make two kinds I like, one is their "Traditional" type which is my favorite and another, whose name I forget, which is basically their "traditional" but also has some Oil of Bergamot (whatever that is) in it which kind of gives it a pleasing lemony flavor. Salada gives me the biggest caffeine kick, which leads me to believe that hopefully it also has the highest concentration of the good stuff.
The only store-bought brand I like (assuming they don't carry Salada at your store) is Bigelow. A bit more smooth and tastier than the Salada, but not as much of a caffeine kick. It looks more pale in the cup, wheras the Salada starts as an opaque bright green and fades to a thick green-tan color the longer it stays in your cup. It leaves the nastiest stains on the cup, too... which leads me to believe that the sh*t is chocked full of the best o' the leaf.
I have heard anecdotally from a few other green tea aficionados that there are really superior green teas to be found out there in loose leaf form, far better than any pre-bagged teas. I haven't done any exploration in that area yet, but perhaps someone who has may very well blow everything I said out of the water here!
Also bear in mind that articles I have read say you need 4-5 cups a day to really ingest a significant amount of the goodies in the leaf. I have 3 cups in the am at home, about the same effect as two cups of coffee, while I watch the news and wait for my minoxidil to dry. Then, right after lunch, a 4th cup is delightful to shake off that post-lunch food coma. It has been a wonderful habit to add to my daily routine.