This might become a long thread. I'd caution everyone to post the SPOILER tag if the quote folk want to share is plot-related.
MIne changes all the time. I'm still discovering new memorable dialogues as I play, even though I'm a 'BG veteran.' I'm currently in Durlag's Tower, so....
Warder: "I am the warrior's curse
I steal his future
I mar his past
The more he has, the less it seems
He becomes a slave
Of glittering things
Yet I hunger... Feed me that which glitters beyond all else."
Once you've given it, he says: "You have completed this riddle. I am AVARICE. I will bring doom upon you in this cursed place."
So many more, so many more. I hope folk participate in this thread.
EDIT: Don't forget this thread, about enemy quotes!:
http://www.gog.com/en/forum/baldur_s_gate/favorite_enemy_quotes