There are so many great voice actors to choose from... Mel Blanc, Lorenzo Music, Don Messick, Paul Winchell, Tress MacNeill, Rob Paulsen, Tom Kenny, etc.
There are a lot of names that haven't been mentioned yet that I would like to add to the list.
How about Walt Disney? Original voice of Mickey Mouse, anyone?
Seth MacFarlane? OK, he uses his own voice for Brian Griffin, but just take a look at all of the voices he created for Family Guy: Peter, Stewie, Quagmire, Tom Tucker, Seamus, etc. Plus, he voices Stan and Roger on American Dad!
Sticking with the MacFarlane shows, I'd also have to include Mike Henry, voice of Cleveland Brown, old pedophile Herbert, maid Consuela, etc.
One final name to add. Another old school name. Thurl Ravenscroft. He sang "You're a Mean One, Mr. Grinch" in the original How the Grinch Stole Christmas!, sang "No Dogs Allowed" in Snoopy Come Home, and voiced Tony the Tiger until his death in 2005 at age 91.