It is no surprise that a game as fondly remembered for its writing such as Planescape: Torment holds many memorable moments.
Depending on the path you play as the Nameless One, there are many things you can do. For example, you can talk the Big Bad into defeat by threatening to commit suicide.
Or by willing yourself out of existance, preventing yourself from ending up in hell.
Also, if you get the "best" ending, you merge with the Big Bad, who is your own mortality, raise all your deceased friends from the dead, and then go forth to stoically face your final death and judgment in the Hells, teleporting to a battlefield full of demons and picking up a weapon before wading into the fight, hearing the echoes of the game's central question: "What can change the nature of a man?".
As an addendum to the above, the fact that, in getting the best ending, you answer the central question. The answer is belief. Belief changes the nature of a man. That a game could answer its Arc Words at all is stunning, to say the least, considering how rarely such questions get answered, but the way the hero says it- "I have seen belief move cities, change hearts, heal..."- is stunning, to say the least. He talks down a spirit of power incarnate and gets to go to his judgment- and through all this, despite the fact it could be a Downer Ending, it manages to be extremely upbeat. The last thing you see is the Rune of Torment, flapping in the wind, no longer a part of the hero. Death is no punishment for him.
The Nameless One can potentially win a battle of wills with the Practical Incarnation.
The Nameless One learns that he can actually revive anyone he likes from the dead. There's also the possibility that he revives the people he came with to face off against the Big Bad.
All of the loyalnote Depending on your alignment, Vhailor or Ignus will be turned against you, and there is no corresponding scene for the alternative characters get one as the Big Bad picks them off one by one - they would rather die than betray the Nameless One and their True Companions. And none of them are even the slightest bit afraid of certain death, regardless of the airs they might put on.
Dak'kon: "I may be bested in battle, but I shall never be defeated."
Annah-of-the-Shadows: He means more t' me than my life."
The Transcendent One: "THEN DIE."
The Mechanus Cannon spell. It shows a brief video sequence of a Wave Motion Gun sized laser cannon firing a continous shot through a portal from the plane of Mechanus to wherever you happened to aim it. And it's utterly devastating in both looks and effects. The makers of Final Fantasy would love it. Most of the other high-level spells qualify as well.
Morte gets one if he successfully uses his Litany of Curses against Ravel. Just to put in perspective: She's an immortal, nigh-omnipotent demon witch from another dimension. He's...a floating skull. And he gets her to throw away every single one of her advantages and engage in fisticuffs...by swearing at her.
When one of the Nameless One's previous incarnations convinces someone that they don't exist. And they immediately cease to do so.
If you decide to beat the Transcendent One to death instead of talking things out, Vhailor can get a ridiculously awesome moment. Vhailor's power goes up as the level of injustice he's fighting is higher. The Transcendent One is part of you, your own mortality. Your original incarnation did something terrible beyond any redemption; the Practical Incarnation's shenanigans apparently pale in comparison, and you are earmarked to go straight to Hell when you finally die for real. The Transcendent One's existence is keeping you from properly dying and being sent to your rightful punishment. Explain this to the revived Vhailor and watch him go administer justice.