My theory of Rapunzel being the first to be kidnapped by Pitch. My reasoning is that Rapunzel would probably be the easiest to break through to. I mean, don't get me wrong. I think Rapunzel is a very strong hearted character. Because just like Hiccup. Her enormous heart and creative mind will surely make her a force to be reckoned with. But though she still would have some fears to overcome because she had been isolated in a tower for so long and whatnot. And that's probably the first way Pitch gets to her. With her fears.
The first fear being Mother Gothel finding her and locking her up in that tower again and the other is her friends forgetting about her and leaving her behind. There are probably more I could think of..but not now xD
But other than that; of course, the others treasure her kindness, but have to work hard to help her come out of her shell. I like to think of Punzie as a graceful, fawn-like girl but still a strong character. I think Pitch would be more of a fool to misinterpret this little lady here, although she may be seen as a weak link at times, she is a determined girl with admirable strength.
AUTHOR NOTE:This is really just came to me in a thought bubble in the back of my head. I don't know if I'll add more to this or not.