Arguably the best RPG of all-time
The two main RPG styles are rarely broken - Japanese RPGs love their part fantasy part sci-fi settings with tween characters and Western ones are quite content with their monogomous relationship with Dungeons & Dragons. Occasionally you get a game that tiptoes out of such a description, but not very often. That in itself is enough reason to take a peek at Planescape: Torment, as while it carries D&D designation it takes that setting (and even style of gameplay) and warps it. With excellent results.
Torment is a role-playing game in every sense, and it's arguably the game's greatest strength. While there is unavoidable combat it isn't frequent, and there are large portions of the game that involve nothing but playing your character in a city full of interesting people. Whether you act good or evil or somewhere in between is entirely up to you.
You play as the Nameless One, a hideously scarred immortal with absolutely no recollection of who he is. This fuels the story, as you are constantly discovering things about his past, and the gameplay, as you cannot die. It also defies any genre conventions, as your character is definitely not a young, handsome, charismatic hero.
D&D peeks in via character classes, as you can play as a warrior, thief, or mage, depending on your preference (and you can switch through at will during the game if you want). Ultimately it makes no difference, although the game is certainly easier to play as a warrior - and being immortal means that those combat-oriented sections are do-able simply by charging the Nameless One ahead and killing a couple of enemies, dying and respawning, and then repeating the procedure. Your party members can be involved in combat, but as none of them are immune to death and healing items are in short supply, there is a strong incentive to simply pull them back most of the time. It doesn't help that very few characters actually have healing spells, and most areas do not allow resting to regain HP. Healing items and judicious use of the Nameless One are key.
All of your potential party members are extremely interesting, well-voiced (voiceovers are a bit too rare, though), and have fairly deep backstories. Some are obviously more prominent than others, but none could be called boring. They range from Morte, a hovering skull, to Ana, a half-fiend, to a perpetually burning mage, walking computer, succubus-turned-cleric, and more. Some join automatically and others require more devious methods to enlist.
The meat of the game lies in interacting with characters and reading text. Sounds boring, doesn't it? It's not. There are many varied quests, usually with alternate ways of completing them (often these are good/evil variants), many areas, and a lot of interesting characters with much to say. Dialogue exchanges are extremely well-written, as are the text descriptions of certain areas. There is an awful lot of writing in the game, to be fair, and often it feels like you're in an interactive novel rather than a game. This is in no way a bad thing unless you absolutely hate reading, of course, as Planescape: Torment is one of the best written games, period. You will care about the Nameless One, be interested in his story and those of your party members, and talk to each and every named NPC in the various areas just to see what they have to say.
The fact that the game lets you be good or evil or in-between is just a bonus. It's not half-assed, either, or as blatant as other games that advertise such a feature. Even simple questions from NPCs can often be answered six or seven different ways, depending if you want your character to lie or otherwise deceive someone. Little things like this creep in to affect your alignment, which in turn affects how people treat you. Even being completely evil is a viable way to play the game and will result in noticably different interactions with people.
If any complaints could be leveled at the game it's that the combat is somewhat unsatisfying (it's very similar to other Infinity-engine games such as Baldur's Gate and Icewind Dale) and the overall length is short. But the length becomes much less of an issue given the replayability, as due to different alignments and the wealth of quests (and alternate endings) you'll certainly see something new the second time. Of course, there won't be any way you can just play the game once, anyway. It infects you.