Arcanum with no doubt. It gave perfect freedom and nearly many many quests had solutions of "evil/good" "fighter/mage/tek-no-logik guy/talker". The theme was the best ever, though it had not the best graphics ever and had bugs, insider is much more important for a RPG.
Yet, Baldur's Gate series has a sentimental value for me. When i played Baldur's Gate I, i was trying to learn about real FRP/RPG and BG1 was definitely a great low to mid level campaign. BG2 was a great mid to high level campaign and Throne of Baal was an ultimate and and a great high to epic level campaign. Yet, BG series were (BG1 was less, BG2 was more and ToB is much more) linear; both in quest solutions and the main gameplay. That is a minus.
Btw. *coughcoughdiabloisactionnotrpgcoughcough*.
Yea, i had to say that. I know i suck.