Because I often write for game fandoms, I am often handed plot lines that require my PC to be something of a Mary Sue/Gary Stu. The challenge for me becomes one of explaining why this one particular person is the only one who can save the world, while at the same time trying to flesh out the frequently cardboard NPCs that follow my PC around.
It's a fascinating challenge to un-Sue that Sue. If you think about it, no hero ever acts alone. There's always something, and that can be the fun part of writing for these fandoms. I like to flesh out the NPCs with back stories that give you an idea of why they're tagging along, and personalities of their own instead of having them serve as mirrors for the PC's actions. I've also been known to credit some of the quest successes to NPCs rather than the PC where it makes more sense for the NPC to have done better. It makes for a richer and more realistic narrative.
Having said that, the anti-Sue is as awful. The lead character who is paralyzed by indecision, or self doubt. The guilt-wracked character who can't forgive themselves for past mistakes, and judges every action by those errors. The one who turns humility into a vice rather than a virtue (which calls to mind another pet peeve completely unrelated to this thread). The self-effacing mouse who becomes somehow competent under duress.
Gamers have a love/hate relationship with Sues, and when we turn writer, we get a chance to see if we can do better. That's the fun part.