For some reason he's the one boy everyone seems to think they know more so than the others.
That's interesting that you said that because I don't feel that way at all. Maybe it's because I've always been a Brian girl, but I feel like Brian, for example, is an open book, easy to read. He is who he is - dedicated to his family and his faith, a comedian, but more conservative than the others in some ways. I don't feel like there's as many different directions to take a guy like Brian, who has basically stayed the same all these years.
Nick, on the other hand, is more of a mystery to me; I never know quite what to expect from him. He's shown us so many different sides to himself over the years, and I think that makes him easier to adapt to certain scenarios because there's really no wrong way to write him. He can be funny, he can be angsty, he can be romantic, he can be an asshole, he can be the hero, he can be the villain, he can basically be whatever you want him to be, and it works!
I see AJ that way too, and maybe that's why he's also more popular than Brian and the other guys in fanfic these days, but Nick is still The Chosen One, so he gets more starring roles LOL.
Howie and Kevin are somewhere in between for me, I think because they're not as out there and open with their personal lives as Brian, but more settled and secure with themselves than Nick and AJ historically have been.
That's my take on it, but that may just be because of who I've paid the most attention to over the years! LOL I'm sure we all see them different ways just based on our own experiences.