Several pieces of advice:
(1) If you want the most antioxidants and the greatest health benefits, make sure to drink fresh-brewed tea, and not bottled tea. I run a tea website, and we have a page that explains why bottled tea is not as healthy...it's not just the added sugar, the antioxidants actually start breaking down after the tea is brewed, so fresh brewed tea (from a tea bag or loose-leaf tea) is much better:
http://ratetea.net/topic/bottled-tea/4/
(2) Some brands are better than others but it's pretty random. And there are few studies that have compared this. Also, it varies hugely from one tea to the next. This said, there is one particular study that I know of in which Stash teas tended to rank more highly than teas from other brands. Stash teas, in my opinion, also tend to be a notch up in quality from some of the other more mainstream brands (Lipton, Tetley, etc.) so I suspect that it's mainly just a function of slightly higher-quality tea. But like I said, it's just one study. You may find the section "Are some teas healthier than others" in this article useful:
http://ratetea.net/topic/health-benefits-of-tea/8/
The study cited which included the Stash teas is here:
http://ddr.nal.usda.gov/handle/10113/1807
(3) Honestly, consider loose-leaf tea. Really. It's hard to buy at the supermarket but it's soooo much better, fresher, and you're paying for the quality of the tea, not the packaging, so it's inexpensive relative to the quality. I like both Chinese and Japanese green teas. One company that's very inexpensive and has a big selections, among the first that I had loose-leaf green tea from, is Upton Tea Imports, and I'd recommend them. If you want to learn more about green tea and read reviews of different companies, I'd encourage you to visit my tea site:
http://ratetea.net/style/green-tea/4/
I hope this answers your question. There is no quick and easy question because there haven't been that many studies. But you'd do well to let your taste buds guide you. The freshest and best-tasting tea is generally going to be healthier for you. And try ordering online so you can get loose-leaf tea.
=) Good luck!