This isn't even tough, man. Her favorite place is the TARDIS. Which isn't on her world, but it used to be. It's beautiful and magical (although she doesn't...actually believe in magic), and it changed everything about her life, from how she saw her world to which world is 'hers'. Even besides the places it takes her, the TARDIS is wondrous all on its own, and so warm and welcoming and comforting. She really does miss the TARDIS.

If she could, she'd stay in the TARDIS--traveling in he TARDIS that is--for the rest of her life. Which...she can't. But if she could.

Also it's the Doctor's 'home'. That helps.