Also, being dependable and capable or above average does NOT make one a Special Snowflake or Mary Sue.
Mary Sues are idealized characters, often having a billion unique/special things about them.
Special Snowflakes are usually unique in some aspect (as in, no one else is like them)
But for both the deciding factor is how they are presented and how others react to them or treat them.
When others worship the ground they walk on or can't stop talking about how awesome they are, when EVERYTHING revolves around them (often in convoluted ways), and all of that feels forced or un-logical.
Best example I can think of would be Ichigo from Bleach. He is both a Super Special Snowflake and a MASSIVE Mary Sue.
A character who starts as a "normal" human, but we find out he's half Shinigami. And a half hollow. And a vizard. And a Quincy. And probably a half fullbrnig or something..
He goes from 0 to super-master-swordsman that can match 500-year old Shinigami who were trained by the finest swordsmen...in weeks. Months tops.
Also, he gains power levels at light speed. Powerful Shinigami train for hunderds of years to unlock their Bankai. He does it in THREE DAYS.
And everyone is fascinated by him (especially the baddie of the week), and he kidnaps every plot threat to somehow be tied to him, despite it having no logical connection.
Warden and Hawke don't really qualify. They might get praised a bit too much and pulled in to deal with stuff they really shouldn't, but all of that is "light" compared to real Mary Sues/SSS.
****
Tinxa is right about one thing - too many titles, accomplishments and skills/powers on a single character, and even the most mundane one will turn into a SSS/Mary Sue.
A good example might be Caphias Cain - a normal human commissar that tries ot avoid conflict as much as possible, but keeps running into it. He gets by with stupid amounts of luck, his with and great sword skills.
And it's all fine and dandy in one book. Or two. But as more and more books roll out, he defeats hive tyrants, chaos space marines, genestealer pariahs, demons, etc.., etc..
He ends up having a list of deeds that would put a Space Marine Chapter Master to shame.