James "Jimmy" P. "Sully" Sullivan (have enough naming options?) is one of the primary protagonists in the Monsters, Inc. franchise from Pixar. Large, furry and blue, Sully has massive claws and fangs, and sports a polka dot colour scheme on his fur. He is the top scarer under the supervision of Henry J. Waternoose III at Monsters Incorporated, and is best friends with Mike. Together, they often come to blows with Randall Boggs.
In most of his appearances, Sully is voiced by the go-to deep-voiced celebrity John Goodman. I think I'm in the majority here when I say that John's casting was impeccable. Sully is a fan-favourite character of his, and you can tell that he has a lot of passion, enthusiasm and energy in the role, simply based on what you hear in his performance.
Brian Cummings voices Sully in most of the video games. Brian does a good job of getting the softness and kindness of Sully, but falls a bit short of having the "power" of John's performance.
In Disney Infinity (in which Mike and Sully are portrayed as their university selves), Joel McCrary takes over the role. Conversely to Brian, Joel nails the gruffness and power in the voice, but doesn't quite get the "charm" of the character, in my opinion.
While all three are good in their own ways, my vote is going with John. He's the original voice, and Brian and Joel do decent jobs trying to fill his shoes.
BONUS OPINION: I'd like to see Fred Tatasciore or Clancy Brown get a shot at voicing Sully. Though I think Clancy would make a better "Old Sully", or even like a James P. Sullivan Sr.