I'm a little surprised that no one has really mentioned Mandalore yet as a favorite voice. The major highlight of the game, for me personally, was John Cygan's performance.
I guess because Mandalore had a relatively small role, and not much dialogue compared to the other characters, he didn't have as much of an impact on most players. But I actually felt he did the most outstanding acting out of *both* of the games, and to me, had the most interesting character.
Especially when comparing him to his appearance in the first game, I got a strong impression of Mandalore's personal growth - in the first game the actual quality of his voice is more gravelly, and he sounds more pessimistic and worn-down. In KOTOR II, I thought he sounded at least five years younger, had a clearer voice (despite the crackly-ness of his helm getting in the way) and sounded more lively, determined, and focused.
::shrugs:: I'm always looking for an opportunity to praise John Cygan's acting, so please pardon me for rambling.