River, Romana and Rose were three people whom the TARDIS could never forget, even if she tried.
Romana, she could never forget, because she was the first true Time-Lady to step inside her after she stole her Thief. That day, the unusually feminine sense of Time had pervaded her system, and in a blink of an eye, it seemed, she and the intoxicating taste were gone. It was like trying something new, loving it, but then having it taken away from you, never to try it again. That was what Romana was like.
Rose, again she could never forget. She wasn't a Time-Lady, by any means, but there was something intimate in the way they had Seen each other. When Rose looked into her heart, she looked into her and that sight, that soul, would forever be imprinted in her mind. That was what Rose was like.
And River. River Song, Melody Pond, her darling Child. She might have been born of Amy and Rory, but the TARDIS was and always would be her mother. And no mother could ever forget her child.
So River, Romana and Rose, after decades, centuries, even thousands of years after they left her halls for the last time, would forever lay in her hearts.
It helped that her Thief loved them just as much as she did.