Like felltablet above me mentioned; Planescape: Torment is not really the best game ever, its got a plethora of problems like glitchyness, crashes on XP, unbalanced gameplay and horrible combat. However, it has the best story ever put into a game, ever. Period. There's no debating this really and if you're trying to debate it it's only because:
A- You never played Planescape so you're still under the impression Final Fantasy or somesuch is high art in storytelling or
B- You simply didn't understand the story or you skipped dialogue and were expecting a convinient cinematic to go about explaining the plot for you in easy to bite bits like most games do. Torment doesn't!
Planescape: Torment taken as an interactive novel is genius. If it were a book, it'd be among my favorites! The story is eccentric and strange beyond all reason, yet still compelling with such an imaginative style to itself you can't help but be drawn in. The characters although similarly strange beyond all reason, are believable. How can a talking skull, a chaste succubus, a body-snatching demon girl, or a possessed suit of armour be believable you ask? Play Planescape and find out. The overall plot is riddled with symbolism, questions of morality and all sorts of tough questions that leave you wondering long after the credits are over as well as many genuinly touching scenes. Not to mention this is one of those games where to really appreciate the plot you'll want to play through the game multiple times, there really is that much in the game it's impossible to see and learn it all in one playthrough.
All that said, as a game, it's somewhat mediocre. Although it uses the same fantastic engine that powered Baldur's Gate and Icewind Dale, don't expect the same level of quality tactical combat here. Planescape's combat I've found is usually tedious and dull. Similarly although not bad, the art design has not aged so well and looks muddy and unclear. Sound is excellent however! Great voice acting and a memorable musical score that differs from the usual high-fantasy orchestral stuff you usually get in rpgs.
Long story short: If you play this game for the story, skip nothing and explore as best you can, you WILL fall in love. In this respect Planescape: Torment is top dog! But if you go in there wanting thrilling gameplay, in-depth statistics or fun tactical combat, you'll only end up leaving dissapointed. Take that as you will.
As for where to find it? Ebay is yer best bet. The game has been out of print for years now, sold terribly. Plus the games developers, Black Isle, folded awhile back. That's about all I think of, I still treasure my old boxed set with the huge ugly orange box (probably half of why the game sold so badly) much and wouldn't part with it for a fortune.
Oh, and if you do get the game, a tip for ya. Max out your int and wisdom scores as soon as you can, then work on charisma. If you try playing the game as a warrior or rogue, you'll only be getting half the story. High intelligence, wisdom and charisma scores are essential. Becoming a mage as soon as possible is helpful, it's not hard to figure out how.