Of those still living and working today, I'd say Frank Welker, voice of more characters than I can name, including Freddie from Scooby Doo, Megatron and a bunch of other Transformers, various non-speaking animals, horny young Spock in Star Trek III and many many more.
Also Peter Cullen, who is mostly known for Optimus Prime, but just has an awesome "movie trailer" voice.