Ranking is more specific to the particular company. At Netflix and Amazon it is obviously very important, as it is part of their recommender systems. At Google, probably less so.

Computer Vision and NLP are both extremely hot research topics. Knowledge gained in either one is highly applicable elsewhere. If you go into NLP I recommend learning about the problems other languages pose, don't just learn NLP for English, if you can.

Personalisation is not something I can say anything about.