I'll start.
My favorite performance is Jaheira, voiced by Heidi Shannon. That unique, eastern-european accent saying "Yes, oh omnipresent authority figure" is unbelievably iconic and also perfectly captures BG's slightly irreverent tone. It's emblematic of the series as a whole, and probably influenced a lot of fantasy RPG voice acting that came after it. For example, I am convinced that Laura Bailey's character Jester in the second season of critical roll was based on Shannon's performance, though admittedly the personalities of Jester and Jaheira are quite different.
And you know what's surprising? Heidi Shannon didn't go on to do anything else. Nothing that I'm aware of anyway. And I have no idea why. Makes me wonder if the voice actors back then knew that they were working on a very influential title, or if they just recorded those 2 minutes or so of voicework for that weird little game company just to pay some bills.
For me it has to be the legend that is Jim Cummins, who voices Minsc and soooo many other things/people - including Winnie the Pooh!