Well… there’s no way to make this easy, really.
Several different perspectives on that complex question, “Who is your User?"… as directed toward the MCP himself.
On the Tron Wiki, both Gibbs and Dillinger are listed.
Looking for evidence of these claims in onscreen canon of the ‘82 movie, there’s clear support for Dillinger:
I may be missing something, but I don’t see any explicit claim of Walter Gibbs as the MCP’s User…
…although he IS explicitly stated to be the founder of ENCOM, so it seems likely he would have been involved in writing the MCP when it was a chess program.
(His own program, Dumont, at least confirms he was there at the time.)
(Interestingly, right after Dumont says he’s "old enough to remember the MCP when it was just a chess program,” his next words are “He started small, and he’ll end small.” Someday I’ll have to analyze in more depth these alternating he/it pronouns for MCP. Perhaps “it” was the pronoun used back in the Chess Days?)
The prologue of the Tron: Betrayal comic offers another perspective on MCP’s creation, but this is narrated by a version of Dumont who…. seems to have vastly distorted memories of what actually happened:
“FLYNN WAS UNLIKE THE OTHER USERS. HE WAS, AND IS, A CREATOR. HE MADE ORDER FROM THE CHAOS OF ONES AND ZEROES.”
…um. I think you transposed the words “chaos” and “order” there, program.
“HIS FINEST CREATION, THE MASTER CONTROL PROGRAM, REBELLED AND TRIED TO DESTROY HIM. THE MCP BROUGHT FLYNN TO THE GRID.”
…setting aside the confusion of what exactly “The Grid” refers to here… even IF Kevin Flynn created the MCP, I cannot imagine anyone calling it his “finest” creation.
(Even by Flynn’s own evaluation, at the time of the '82 movie, his infiltrator program Clu1 was “the best program ever written.” Sorry, MCP, that sweet little User’s boy is better'n you.)
But anyway.
The Tron Wiki acknowledges that Dumont’s narration here is incorrect:
But we do, at least, know that Flynn interacted in some capacity with the MCP when it was a chess program:
Could he have worked on beta-testing and/or debugging the chess functions?
If not the original programmer, he may at least have been involved in development of the MCP, in some capacity.
But now let’s look at the MCP’s own viewpoint on its origin.
When Dillinger says “I wrote you!” this is how MCP responds:
He doesn’t contradict Dillinger’s claim to have written him, but he dismisses it as unimportant, in light of how far he’s come since Dillinger did any of that writing.
Then, when Sark makes a similar admonishment:
This time, the MCP does contradict him, claiming he doesn’t have one singular User. Rather, he considers himself a collaborative project so vast that any individual contributor is unimportant.
So, even if Dillinger and Gibbs– and maybe Flynn– were all involved in developing him… from his own hubris-soaked perspective, none of it even matters.
Is this the face of his original User?
Who cares.
He started small and he ended small.