Alright, since i'm the one who started this thread, I might as well say who my favorite is. Firstly, I just wanna say, Fran Brill is an incredible performer. She's so versatile and you just completely forget that Zoe and Prairie are played by the same person. I absolutely adore Zoe, but i'm definitely going with Prairie here.
Prairie Dawn is not only my favorite Fran Brill character, she's one of my favorite characters of all time! As Drtooth said, when you get two characters together who can really annoy each other, that's the making of great comedy. Sure Ernie and Bert are a fantastic duo, but Prairie and Cookie are probably one of the most underrated comedy duos ever. That's truly what makes Prairie jell so well with the show, the whole show can drive her crazy because of the shear illogical-ness of it all, but much of the time she's still playful and able to be silly with them. Not only that but, she often just enjoys being quiet and reading. I relate to her so much because I was always an introvert who seemed to enjoy reading far more than my peers, I was quiet and shy and always felt slightly more mature then a lot of kids. I didn't feel superior to them at all, it's just the way I was.
But another thing I enjoy is that she's a girl but she doesn't want to be a ballerina or a princess, she wants to be a director and a writer and a composer and she truly knows how to take charge. And no one ever gets annoyed or calls her bossy, they just listen to her because they know she knows what she's doing.
That's why I love Prairie Dawn, because she's such a well rounded character and because, but in some ways she's like me. And i'm sure she relates to a lot of kids who feel like they don't fit in with the crowd. But once again, I still love Zoe to pieces, she's one of the more multi dimensional female characters on the show and she holds a special nostalgic place in my heart, I really wish they'd use her on the show more. And Fran Brill is great at doing two characters who couldn't be more different from each other.