Don Messick was the original voice of Scooby and provided the voice for many years until his death. See the entire list of the voices of Scooby Doo.
Scooby was not Messick only character or even the only dog for whom he provided a voice. In addition to Scooby, he provided the voice of Scrappy Doo, Muttley in the Wacky Races, Astro, and Bandit from the Johnny Quest series. In fact, he was legendary as a voice actor. Other characters that Messick voiced include Boo Boo Bear, Ranger Smith, Atom Ant, Papa Smurf, Bam-Bam Rubble, Richot Rabbit and a slew of others. Messick passed away in October 1997, when Scott Innes inherited the role.