Planescape: Torment is the best game I ever played, mostly because of the superb dialogue and fascinating stories. It brought the Planescape setting to life and filled it with intricate plots, strange characters (Morte, Fall-from-Grace, Ravel) and amazing locations (The Brothel of Slaking Intellectual Lusts, Drowned Nations). The writing was better than the vast majority of fantasy novels and it propelled a story that was exquisite in its morbidity.
It also had plenty of cool monsters, weird magic items and the great Infinity Engine combat system. I haven't seen anything to better it since. It not only made you explore the setting but also your inner self and the motivations of all conscious beings. It was fantasy philosophy made flesh and it did all that so well.
"What can change the nature of a man?"