Why should an ending where Ciri becomes Empress be the best one, I would think the one where she becomes a witcher and sets out on her own would be the best one, seeing it's what she always wanted to do
Not really true. She always wanted to help people, if she didnt meet her father then being a witcher and killing monsters fulfills that yeah, but if she does meet with her father, she can do much more to help people being an Empress. So both endings are what she wants.