I am quickly showing up in here to say, Earl Grey has a freakin' ton of caffeine, and should never be drunk before bed, unless you have some sort of mad caffeine tolerance, in which case, bully for you.

Hot
Chamomile is excellent for relaxing (mighty leaf tea does an incredible job with their chamomile citrus blend. tastes like cake. Celestial Seasons or whatever does just a plain chamomile () and their special Sleepy Time blend, which is pretty good.) Awake by Tazo is a black tea mix with a decent amount of caffeine. PG's PG Tips are freaking DELICIOUS as the standard milk+sugar+hot tea, as is Tetley's British Blend (here in the US) or "tea", as the brits call it. Gunpowder Green Tea by Red Blossom is the most delicious green tea i've ever had, but most of the "gunpowder green" teas sold in stores are pretty decent. Oh, and Earl Grey and all its variants are so delicious that it should be a staple on your tea shelf (I drink a variant of Earl Grey called Dorian Gray, which is regular Earl with a caramel flavoring added to the tea leaves.).

Cold
Vietnamese tea (standard black tea mixed with sweetened condensed milk) + tapioca pearls is delicious. On a related note, mango-peach black tea with tapioca pearls is the pinnacle boba-filled teas. Peppermint, spearmint, or a blend of mints tea is ridiculously refreshing when served cold. Please don't drink Lipton. It's not worth your money or your time. If you must make some sort of sweet southern iced tea, may I recommend Luzianne's blend. Made the best and strongest sweet tea I'd ever had.

Hot or Cold?
Chai (black tea with cloves, cinnamon, cardamom & other spices) fits into this category, 'cause it's delicious both ways. I prefer mine iced, but that's just because I live in a tropical climate. Hot chai is like drinking everything that makes pumpkin pie delicious. You can sometimes find green tea chai, and I highly recommend the experience.