FiringSquad has ranked the top ten single-player and multiplayer PC games of all time, with Planescape: Torment coming in at #4 in the single-player category. An excerpt to follow:
Torment was ahead of its time, but unlike Allegiance, not in a bad way. One of the greatest compliments I can pay to the Black Isle team that developed it is that they out-BioWared BioWare. As good as Baldur's Gate II is, few who've played both games can say that it's the better game. Yes, the combat is better. Yes, it sold better. It had multiplayer too, and a great expansion pack. But Torment had the best story of any game, ever. "Flaws" like pre-built characters were strengths as you played the game and discovered the mystery behind the Nameless One and his strange relationship to the floating skull, Morte.
The mystery and the character developments were interwoven with each other and the game world. Even the famed Ultimas (many of which could rightfully argue their spots onto this list) didn't feel as complete to me as Torment. Oh sure, they had their day and night cycles, and NPCs had schedules, but Torment was so clearly a labor of love, filled with painstaking detail. True, it lacked many of the cooler features of Baldur's Gate II, like getting a fortress for your character or the millions of sidequests with their cool gear, but Torment was more focused and delivered a far better story.