Frank Welker. I'm still being surprised by the volume of stuff he's done, both in cartoons and live action. (Seriously, he's even done animal noises for live action TV.)
And then there's times when I'm looking back on an episode of the original G1 Transformers and there's a bunch of Decepticons onscreen and I realize often it's Frank Welker talking to himself... and himself... and himself. (Megatron, Skywarp, one of the Constructicons, Soundwave and I think ALL of Soundwave's tapes. All one guy!)