He’s everywhere. And he does good work.
Mark Hamill
Kevin Michael Richardson. He has a very prolific body of work, including the Joker on the Kids’ WB series The Batman and as Barney Rubble in The Flintstones: On the Rocks. He currently appears on The Cleveland Show and might I add, does a pretty mean Isaac Hayes impression. He played Rockefeller Butts on the short-lived ABC series The Knights of Prosperity.
Geez, this thread is becoming more of a Sgt-Pepper’s-Album-Cover of voice actors than a Mount-Rushmore of voice actors. (And it causes me to wonder: How do you carve people’s voices into the side of a mountain, anyway?)
I going to interpret the OP to be asking, What four (give-or-take) people are the all-time greatest voice actors, judging by who has created the most iconic and widely-recognized voices.
By any standard, Mel Blanc has to be there. June Foray, Frank Welker, and Daws Butler are worthy candidates, though they aren’t as widely known. Among current stars, Dan Castellaneta, Seth MacFarlane, Tress MacNeille, and Billy West are at least worth considering. And while I think of Frank Oz more as a puppeteer than a voice actor (voice acting being a subset of what a puppeteer does), it’s true he has created quite a few very iconic and widely-recognized voices (as have some of the other Muppeteers).
I think, like comics, we need to divide into eras. For the Golden Age (roughly 1930-1965, which covers radio, movie cartoons and the first era of TV cartoons) I’d go with Blanc, Foray and Butler. The fourth spot is a tossup with Conried and Bea Benaderet, among others.
The Silver Age starts in 1987 with the first Simpsons shorts. I nominate Welker, West and MacNeille, and about eight others for the final spot.
Paul Frees’ best-known character is Ludwig von Drake, but he did a host of others, as well as lots of voice-overs, dubbing, and narration (including many Disney park attractions).
Let’s not forget Clarence Nash. He may have essentially only done one character but it was Donald Duck. Nash’s voice was a major part of the character. And Nash did Donald’s voice for fifty years, from 1934 to 1984.
Jack Mercer’s another possibility. His main character was Popeye but he also did a bunch of other characters.
Orson Welles, mainly for his radio work. And even though he’s not technically an actor (although “A Boy Named Sue” comes pretty damned close), Johnny Cash!
I vote for Paul Frees. Being a fan of older films, I hear his voice a lot. (And Spike Jones’s version of My Old Flame, which features Frees impersonating Peter Lorre, is one of my favourite Spike Jones numbers.)
John Di Maggio – Bender, Schnitzel, Jake, Dr. Drakken, Marcus Fenix
Vhut? No love for Pottsylvanian moose-and-squirrel chasing spy, Boris Badenov?
June Foray’s birthday is tomorrow (18 Sep). And she was born in (wait for it, wait for it) Springfield! Spooky…
She was born in 1917, so she is older than Mt Rushmore.
Not just the carvings, the geology 
Chuck Jones once said, “June Foray isn’t the female Mel Blanc. Mel Blanc was the male June Foray.”
Excellent choice; that’s who I came in to mention.
Well, there’s a choice between those who have voiced a few absolutely iconic characters, or those who have been very prolific. (Mel Blanc was both.) I can think of a few characters where June Foray was fantastic, but I don’t think of her as being nearly as prolific as Tress MacNeille. On the other hand, MacNeille’s only really iconic character is Dot from Animaniacs.
Picking just four would be tough.
How about Thurl Ravenscroft? In addition to having a fabulous voice (and resume), he wins the award for best name ever…
I particularly like Cree Summer and Noelle North, but I probably wouldn’t put them too high on the list.
How about Grey DeLisle? She has done
Asajj Ventress in Star Wars: Clone Wars
Mandy in The Grim Adventures of Billy & Mandy
Azula in Avatar: The Last Airbender
Frida Suárez in El Tigre: The Adventures of Manny Rivera
Vicky and her sister Tootie in The Fairly OddParents
Sam Manson in Danny Phantom
Frances “Frankie” Foster in Foster’s Home for Imaginary Friends
Yumi in Hi Hi Puffy AmiYumi (in Japanese!)
She’s also he current voice for Daphne Blake in the Scooby Doo franchise. Here’s a link to her wikipedia entry.
My vote is a tie between Alexander Scourby (although I grew up listening to him narrate National Geographic specials, not the Bible), and Peter Thomas. Thomas narrated film strips I saw in high school, and I was mesmerized by his voice. It wasn’t until many years later I found out his name.
Among the more recent times I’ve heard his voiceover was on PBS, for the program Nature, when he says “Funding for Nature provided by…”