Is it just me, or was the twist from Kotor extremely underwhelming? I've been surprised by twists in the Sixth Sense, The Usual Suspects and Fight Club even while the former 2 were spoiled to me beforehand, so me correctly predicting Kotor's Big Twist was a first.
Apart from that, both the buildup and the moment of revelation were very badly handled in that game, so the impact of it was reduced to zero.
Compare that to the twist in Bioshock. Not Pulitzer-worthy in terms of story, but the way it was handled made for a huge impact leaving me feeling thoroughly screwed. In a good way.
I could have done without the flashes of those photograph-memory-thingies tho, they were kinda distracting.
As far as endings go, Portal had one of the greatest finales I've experienced, and I felt both elated and hollow for days after finishing that game.
I've been told Anachronox has a great twist and ending too, but I've never gotten that far into the game.