Just thought I'd ask this before I pop off to the parents for the rest of the week. Is there any original characters you have written that you feel that you've shown too much favoritism to? Basically, are there any characters that, to you, approach Mary Sue territory or you are afraid will if you're not extra diligent about how you use them? Have you ever written something and realized that it turned Mary Sue-ish and had to re-write it?
The problem with "Mary Sue" is that it's become so broadly defined as to be practically useless as a term of criticism. Fans basically use it for any character they find unappealing or excessive. So I'm not sure it's useful to approach the question in those terms. If you worry about whether someone will label a given character a Mary Sue, you'd never get any writing done, because there's bound to be someone who sees them that way. There's a perception today that any guest character who steals the spotlight from the leads is automatically a Mary Sue, which is why you hear the charge leveled against Piper in Dreadnought! and Evan Wilson in Uhura's Song. But the fact is, it was common in episodic TV from the '50s through '70s to aspire to an anthology-like approach -- to have the regular characters travel from place to place or move from case to case and thereby get acquainted with the guest characters and their problems which would be the focus of each week's drama. TOS itself did this to an extent, at least initially -- focusing on Mitchell and Dehner in the second pilot, Harry and Eve in "Mudd's Women," the Romulan Commander in "Balance of Terror," Charlie in "Charlie X," etc. In the end credits, the central guest stars were billed above semi-regulars like Doohan, Takei, and Nichols, because the paradigm of '60s TV gave so much importance to the featured guests. And of course in modern Trek Lit, we have cast changes and story evolution that leads to original characters being added as new regulars, not just guest stars. That's just the same kind of process that led to Chekov being added to TOS, Pulaski, Guinan, and Ro to TNG, Worf, Winn, and Damar to DS9, etc. So a guest star taking the spotlight is not automatically a Mary Sue -- it's just a common trope of episodic storytelling from the era. A Mary Sue is that trope done badly. So basically what you're talking about is just the difference between writing characters well and writing them poorly. The mistakes that produce Mary Sues are the same ones that produce bad characters of any sort -- self-indulgence, unnatural characterization, making characters too perfect or one-dimensional, telling rather than showing, etc. So avoiding favoritism is just part of generally trying to write reasonably well. There's nothing intrinsically wrong about a character dominating a story if it works, if there's good reason to focus on them, and if the other characters aren't mischaracterized in order to let the character in question dominate without deserving to. Ultimately, I think we just try to focus on making the story work the way it needs to, and letting the characters evolve and interact naturally.
I can understand Evan Wilson, since I always felt the exchanges between her and kirk/spock felt pretty off. Didn't matter in the greater scheme of things since other then those exhanges she was a good character(and avoids those charges in every other respect), and the book was fantastic.
While reading Uhura's Song, I totally felt that Evan Wilson was a poorly-written Mary Sue character, until the final revelation made everything click into place. In retrospect, she's an interesting character and one that I kind of wish had popped up again.
Yeah, "Mary Sue" may be in danger of joining "jumping the shark" as a term that has outlived its usefulness. Beyond that, though, I worry less about playing favorites than about occasionally neglecting a character or two. True confession: In retrospect, I didn't give Sisko or Janeway enough to do in my DS9 and Voyager books. There was a period where I found captains and authority figures hard to write, so I tended to focus more on colorful supporting characters like Quark or Odo or Kes or B'Elanna or the EMH. (Hmm. Looks like I was also favoring the exotic, non-human characters to some degree.) Conversely, most of my TOS books are very Kirk-centric, to the extent that (as on the original show) Sulu, Uhura, Scotty, and Chekov tend to be underutilized. I tried to make up for that in my upcoming book, The Weight of the Worlds, where one of my primary goals was to give Sulu and Uhura some time in the sun. Poor Chekov still gets the short end of the stick, but maybe I can do something with Pavel in my next book . . . .