Hard to pinpoint, Narmeleth was certainly memorable, but I think we spent too little time with her after the goth-phase. Shame really, I think she could have added a different energy to the story as a recurring character. Mordirith was pretty cool. Maybe a little lacklustre in the final confrontation, but certainly the #1 original villain of this game. The Iron Garrison members Bosi, Brogur, Broin and Bori have always been compelling to follow. And then there are the rangers Candaith, Calenglad and Golodir. I liked Igash, despite his limited role. He was more imposing than most other villains I've encountered in this game. The voice, look and presence stood out. That's about it from the top of my head. To pick just one though, hard to say..
Nearly forgot Draigoch, now he's cool, he put all other dragons in the game to shame. I take it, he, along with Smaug are referenced in one of the final Gundbad instances where they mention 9 great dragons?
"...None of us would join the Grey Company if we felt its errand was not important enough to brave those risks. For my part, I will not give in to fear of the unknown. We all have our role to play, and I hope only that when I have played mine, the world will have been better for my having been in it.