hmm, as far as excellence in a series go there are too many in a tie for me to list, and most of my favorites have already been listed...
as to what I'd call the best... as in, a scene where the acting actually affected me emotionally... well, I'm probably in the minority, and feel free to mock me for the admission if you like, buuuut....
Metal Gear Solid 2. not the whole game, god no. as competent as the voice cast is, the dialogue they're working with is just too far beyond the line for it to win that kind of accolade.
No, I refer specifically to Otacon's performance in the aftermath of the final Vamp battle. I won't spoil just in case anyone has failed to play the game who may want to, but anyone who has played knows of what I speak.
In particular, the scene toward the end of the event, where he's by himself looking at the bird, who recites the only phrase it was taught to say, and he just breaks down sobbing. I did the same when that scene hit, the first time I played.