That would be an insult to the cartoon Snow too. If they made Snow hopeful that would be one thing, but as you said, she was defeatist and needed to be cheered up whenever things got bad. They kept flip-flopping her between "I'm Mary Margaret here!" and "I don't want to be MM anymore!" to the point you'd think SHE had split herself in two. I think out of all the characters, Snow was the most destroyed character.
Second place was Henry, since he didn't have as far to fall. He was a smart, pretty well-defined kid in Season 1 who saw Regina for who she really was. He was the first casualty on the Writers' block by having him declare false equivalencies between what Regina did and what the "heroes" (Emma lying about his father and Snow defeating Cora) were doing.
I would say third place was Belle, except I didn't find her that great even in "Skin Deep". She was hollow as a character from the start since a big part of Belle was her relationship with her father and they ignored that completely on this show right from the beginning (because this show values family relationship... right).
I was afraid they would ruin Emma in 5A, but they actually didn't. She didn't do anything that was remotely evil in my books. Her ruination came in Season 6. It's not that she became unlikeable. She just became completely flat and devoid of any spark or personality.
I'm not sure if they put enough energy into Charming to really destroy him, except as a casualty of the Snow character assassination.
For their favorite character, the Writers ruined Regina as early as 2B, by having her kowtow to her mother and acting like she erased all her evil deeds by agreeing to sacrifice herself for Henry. Not to mention making Regina into a mass murderer, which was totally unnecessary. And every time they had the "heroes" make excuses for Regina or bend over backwards to accommodate her while Regina responded like an ingrate (oops, I mean sassy strong woman), they actually undermined Regina as a character.
Meanwhile, Rumple was ruined beyond hope in 4A, less so with what he did to the heroes, but moreso because they had Belle constantly making excuses for his horrible behavior and then writing the story as if Rumple was actually good inside and deserved Belle's love. That disgusting unearned "romance" just made Rumple less and less bearable, especially when he told Belle he wouldn't change but continue to pursue her resulting in the gag-worthy "Up Someone's Back End" episode.
I think the only character who wasn't ruined by the end was Hook, though not for lack of trying, what with revealing he killed his father and left his stepbrother an orphan, and then deciding he killed David's father. At least they had the character consistently accept responsibility for his actions and try to atone for his crimes.
The mark of a bad show is by the end, you find you don't care about any of the characters anymore because they had lost all their core characteristics or had become unlikeable.