If we are being serious, the fact is that all of the mane 6 and even Starlight are remarkably high quality females.
But twilight sparkle in particular, what makes her seem the "most" special i guess you could say, is that her defining trait is that she is intelligent, the others aren't stupid but they aren't as resourceful with the knowledge like twilight, this sort of resourcefulness and stoicism is exceedingly rare even in males, you are talking maybe one in a thousand in females.
To compare, you've probably met women who are strong, likes sports, is shy or who like to party or play dress up, but you have probably never met a girl who is not only very cute and kind, but is also legitimately intelligent to the impressive degree that twilight is, not to mention she has a very good work ethic which is almost as uncommon as intelligence in both genders.
Her personality is just the least common and the most sought after, especially when it is in combination with her other physical and mental traits that make her a female example that is so remarkable that it is practically unrealistic in actual real human females. In order for a real girl to be like twilight, she would have to hit the genetic lottery like a train AND also have an unparalleled upbringing, this basically never happens in real life and even if it does other factors usually ruin people, pretty people never "need" to be smart, smart people are usually to consumed by work to be pretty etc. Funny enough, people who manage to pull off both against the odds are often referred to as unicorns, which just seems fitting does it not?
Twilight is literally the embodiment of the perfect yet unattainable girl.