Methinks this is more difficult than the 'most evil' question.
I would definitely go for Imoen. Good natured, loyal to a fault, honest and kind to others. She treats evil NPC's more with wit than outright hostility.
I have doubts about:
- Aerie. Her goodness seems to be the result of abuse and trauma, which makes her timid and desire order and safety. She has a great desire to do good, but is it really the result of internal morality?
- Minsc. The man is not just dumb, he is insane. He revels in bloodshed, slaughter and violence, as long as the victims are 'evil'. Very chaotic indeed, but 100% good? Hardly in my book.
- Sir Anomen.
Starting out as lawful neutral, in my opinion he stays that way. He just becomes even more arrogant and lawful to the max. He doesn't really seem interested in 'wasting time' on side quests even when they are plainly good.
- Mr. ToB.
The shift from CE to CG just seems to much. His banter is still evil after the alignment shift and his epilogue is definitely CN