Contributor Jishai at visual data site Dadaviz has made this amazing graph depicting the characters played by the core cast of The Simpsons.
As you can see, Dan Castellenata is the voiceover champ, playing 26 characters:
Homer Simpson, Grandpa Simpson, Krusty The Klown, Barney, Chief Wiggum, Groundskeeper Willie, Hans Moleman, Sideshow Mel, Itchy, Kodos, Gil, Squeaky Voiced Teen, Mr Burns' Lawyer, Rich Texan, Louie the Mobster, Bill (of KBBL's Bill and Marty), Arnie Pye, Mr Teeny, The Yes Man, Scott, Leopold, Rabbi Hyman Krustofski, Charlie (from the power plant), Gary (Homer's nerdy friend from college), Santa's Little Helper, Frankie the Squealer.
Hank Azaria does 24 voices:
Apu, Moe, Chief Wiggum, Comic Book Guy, Lou, Carl, Dr Nick Riviera, Snake, Professor Frink, Kirk Van Outen, Luigi, Bumblebee Man, Captain McCallister, Superintendent Chalmers, Cletus, Disco Stu, Duffman, Old Jewish Man (at Springfield Retirement Castle), Drederick Tatum, Legs, Wiseguy, Akira, Doug (the nerd), Johnny Tightlips.
Harry Shearer is behind 23 characters:
Mr Burns, Smithers, Ned Flanders, Principal Skinner, Otto, Lenny, Reverend Lovejoy, Dr Hibbert, Kent Brockman, Jasper, Eddie the Cop, Rainier Wolfcastle, Scratchy, Marty, Dr Marvin Monroe, Kang, Herman, Dewey Largo, Judge Snyder, Sanjay, Benjamin (nerd), Jebediah Springfield, God.
Tresse MacNeille voices 12 roles:
Agnes Skinner, Lindsay Naegle, Brandine, Cookie Kwan, Crazy Cat Lady, Mrs Hibbert, Dolph, Mrs Glick, Lunchlady Doris, Ms Albright.
Pamela Hayden also provides 12 voices:
Milhouse, Rod Flanders, Jimbo Jones, Janey, Sarah Wiggum, Malibu Stacy, Patches the Orphan, Ruth Powers, Wendell, Lewis, Richard, Ms Pennycandy.
Nancy Cartwright plays Bart Simpson and eight others:
Maggie Simpson, Todd Flanders, Ralph Wiggum, Kearney, Database, Wendell, Lewis, Nelson.
Maggie Roswell is six people on the show:
Maude Flanders, Helen Lovejoy, Ms Hoover, Luanne Van Outen, Princess Kashmir, Mary Bailey.
Russi Taylor also voices six roles:
Martin Prince, Uter, Sherri, Terri, Wendell, Lewis.
Julie Kavner's main role is as Marge Simpson, but she also voices Selma, Patty, and Jacqueline Bouvier.
Yeardley Smith is Lisa Simpson, but has also served as Maggie Simpson's voice.
Finally, there was Marcia Wallace, who voiced Mrs Krabappel. Wallace died in October 2013.
