Tea. Any and all teas, depending on mood and circumstance.
Oolong tea (perhaps the original and best version) deserves to be far more popular than it is. Not quite so vegetably as green tea and not quite so full of polyphenol rings as black tea, it serves as a nice filler to make hot water more interesting. I drink more of this than can possibly be healthy, and usually have a pot of it by my computer at all times.
Russian tea kicks all Heinz 57 varieties of arse when you're in the mood for it, but it's also incredibly strange as far as teas go. For a basic foray into russian tea go and buy a slightly smokey chinese tea (perhaps Keemum) and brew it about ten times as strongly as the instructions tell you. Now put the resultant liquor in the bottom of a cup and pour about six times as much hot water on it (believe me, the resultant concoction is quite different than if you put all that water in while brewing it). Make a very thin slice of lemon and float it on top of your tea then put a small dollop of your favourite berry jam in the cup (it would be a sweetened black cherry in syrup, but the odds of you having them lying around are pretty low). That's not the only 'authentic' way of preparing russian tea, but it gives you a pretty good idea of what it is, which is curiously compelling, in the right circumstances.
When I'm out and about, of course, I usually end up with black tea or green tea, since everywhere you go you can usually find these nearby. I've yet to find a black tea that is improved more by sugar than by milk or lemon, but that's not to say they don't exist. Although I do wish catering companies would stop putting a teabag meant for a teacup into a damned pint mug, pouring milk on top and calling it a cup of tea.
Mint tea is another one of those speciality tea cultures, like Russian tea, that has a special place in life and fills it perfectly when the circumstances arise. It's made by pouring a decoction of green tea into a pot with mint leaves and enormous lumps of rock sugar (as in, a chunk the size of your thumb for every teacup's worth of tea you intend to serve). Traditionally it's then poured from a great height into small glasses, the contents of which are put back into the pot several times (to facilitate mixing and aeration) before it is served. It's sweet, minty, comforting and the perfect drink to serve to long lost friends while you catch up on a lazy summer afternoon.
Herbal tea is an oxymoron and people who think 'tea' can mean any sort of tisane, infusion or decoction of bits of fruit, bark or leaves from anything but tea plants is just plain wrong. Shut up, you're wrong. No, don't argue. Tea is a sort of tree, or a drink made specifically from that sort of tree, it's not a synonym for 'beverage made with planty-bits'. *Ahem* sorry about that...
Anyway, herbal tisanes are often quite tasty, and certainly have their place, but I'm very glad the OP saw fit to separate them from tea. I particularly like a strange concoction I was introduced to by a friend in Devon, England, which is made with ground olive leaves (strained out of the liquor that you actually drink, I hasten to add). It's utterly unlike most other herbal infusions but very tasty in it's own way. I've yet to work out why anybody, anywhere on the planet, would drink valerian tea, which I spotted in a health shop, since valerian smells like cat piss and tastes awful (probably also like cat piss, but I'm not in the habit of drinking it, so I can't be sure).
EDIT:
Oh, I forgot all about spiced teas. Usually they simply get called Chai ('Tea', in Indian) but they're properly called Masala Chai ('Spiced Tea', oddly enough). I like the more generally familiar version, with 'warm' spices and vast amounts of sweetener. Blends with black pepper are, to my mind, a more pleasing beverage than those with ginger, but that's purely personal preference. Kashmiri blends of spiced tea are to be praised if you find somewhere that does a good one, but the odds of that are slightly lower than the odds of Yahtzee calling a game 'an unmitigated delight in every way' without first taking psychotropic drugs and gouging his eyes out with forks.