Well, as this seems to be the popular subject of debate, I figured I might as well give a short, sweet, and to the point statement about Fluttershy. I'm not going to bog this down with images or other extras, I'm just going to give the information that I think is relevant.
"Attractiveness" is such a broad topic that most anything can be made to fit the category. Thus, I'm going to try and size this down: What is "attractiveness"? Is it aesthetics? Character? Mystique?
I think it's all of the above, fashioned into one idea: how relatable the character is. Are they your type? Or do they clash with your personality? (I'm going to ignore aesthetics altogether, as favorite colors and favorite mane designs are purely on a person by person basis.)
I heard this description once, and I think it works well: "She cooks, she cleans, and she's good with children. WHY CAN'T YOU BE REAL?!?"
All of the above is true - Fluttershy, although a bit socially awkward at times, has been shown to be neat, tidy, and gentle. She's kind when others need it, yet assertive if the situation requires it. She's never harsh towards anyone. She takes the time to tend to her friends and their needs, even it it's at her own expense. She's willing to give others (like Discord) the benefit of the doubt, even when all odds suggest that it's the most illogical course of actions.
Isn't that what people look for in one another? A soul who would like you for who you are?
She isn't perfect by any means. She can be a bit of a pushover at times, she's a bit reluctant when it comes to trying new things, and heck - she's even afraid of her own shadow.
But at the end of the day, if you're there for her, she'll be there for you. That's the greatest thing I could ever ask of anyone.
Now, you're probably asking: "Cloud, what about the rest of the characters in the poll?".
Well, I think they're just as awesome.
They're all great characters - I voted for Lyra multiple times. However, at the end of the day, Fluttershy's character is grounded in canon material. Her character will not change, and she can never be made out to be something different than what's been put in front of us. When the choice the choice must be made, I have to opt for the kindness that will not change. Perhaps my favorite will change someday, but it would be a sad thing to imagine grand and complex character traits for someone, only to have my fanon ideas smashed by a sudden canon reference.
Now, go vote for Fluttershy! XD