I have real trouble deciding for this one. There are so many. My four favourites are Mel Blanc (who voiced most of the Looney Tunes Characters), Daws Butler (who voiced many Hanna Barbera characters such as Barney Rubble and Huckleberry Hound), Don Messick (who voiced many Hanna Barbera characters such as Scooby-Doo and Muttley), or June Foray, (who voiced the Looney Tune's Granny, and Witch Hazel).
I think I would have to choose Mel Blanc, just because of the sheer variety of different voices he was capable of producing. You would never be able to tell that he was both Bugs Bunny and Daffy Duck.