Story of PST is very complex and emotional, NPCs are very developed and you can visit many planes and get in contact with many fractors. All of these things imply that the gameplay of Torment is very interesting and that it is ONE big experince...
...but what is your biggest personal experience during the playing of PST?
[FOR EVERYONE - FOLLOWING LINES AND POSTS CAN INCLUDE MINOR/MAJOR SPOILERS SO BE CAREFUL IF YOU HAVE NOT PLAYED/FINISHED PST YET!!!]
Personally, I can't decide for one - there are many interesting characters, places and situations...
- dialog with Ravel (btw I've not seen any longer dialog in any game than this one )
- Dak'kon's Pronouncement of Two Deaths As One (and whole Circle of Zertimon stuff... Githzerai history is nifty explained there...)
- Morte and his confession by Pillar of Skulls
- Ending dialogs with party members, especially that with Fall-from-Grace with quote: "Time is not your enemy... forever is."
- Turning of Trias back on the "good" side
- Opening of the portal to the Fortress of Regrets
- Any dialog with Deinarra
- Any restored memory of Nameless One
and
- Deionarra's legacy in sensory stone (that was probably my biggest emotional experience from PST)