The unsung talents of the show are Harry Shearer and Hank Azaria. They do more voices than any of the others, and are excellent comedians beyond just The Simpsons.

Hank Azaria

Apu Nahasapeemapetilon
Moe Szyslak
Chief Clancy Wiggum
Comic Book Guy
Lou
Carl
Dr. Nick Riviera
Snake
Kirk Van Houten
Captain McCallister
Bumblebee Man
Superintendent Chalmers
Professor Frink
Cletus
Dredrick Tatum
Pyro (a.k.a. Chase)

Harry Shearer

C. Montgomery Burns
Waylon Smithers
Ned Flanders
Principal Skinner
Otto
Reverend Lovejoy
Dr. Julius Hibbert
Kent Brockman
Jasper
Lenny
Edde
Rainier Wolfcastle/McBain
Scratchy
Kang
Herman
George Bush
Judge Snyder