Oh geez. This is a hard question.
Gayle and Bianca are both very special to me, because they were dreamies given to me by my girlfriend and my BFF, respectively.
If they left, I guess I could cycle 16 and get them again, but it just wouldn't be the same if they weren't from my loved ones' towns. U n U
But I think I would want to stop Tangy from leaving. She's the last remaining of my starter villagers, and my town just wouldn't be the same without her running around!