I think Maria deserves this spot on most people’s lists because she’s not a monster, or a perverted vision of the conscious side of the town.
She’s a real person, made by Silent Hill.
Like if you play the “Born from a Wish” segment of Silent Hill 2: Director’s Cut, you learn that she’s a real person, she has her own ideas and dreams and thoughts, and she’s painfully aware of her own mortality.
Which considering she exists to toy with James and die repeatedly and brutally and horribly is not only a testament to how far the town will go, but also to the severity of the guilt that James feels.
Whereas most of the other side characters seem to be two dimensional (with a few exceptions) Maria is a surprisingly well developed and realised character in her own right, not just a figment of James’ Psyche but a real person, and that makes what happens to her throughout the game probably the most horrible thing to happen to anyone in the entire series.