Starkiller or The Son. As they are characters he partly originated.
I'd vote Darth Maul, but I feel he's a bit over hyped because we've heard his voice the most. And even when we hear him as Emperor Palpatine, which is cool until we hear Ian McDiarmid record over Witwer's lines in the first/second episode of Rebels S2, which of course is very hard to not hear in future viewings. Regardless of all that, I do love the characters and stories told thus far.
Additional voices, um...which one!? Lol And Narrator is new to me, so no comment there.