Which is more correct: "بارك الله بك" (May Allah bless you - literally, "with you") or "بارك الله فيك" (May Allah bless you - literally, "in you")?

The most accurate phrase to use when supplicating for someone is "Barakallahu feek" (May Allah bless you), as was narrated from the Prophet, peace and blessings be upon him, in his supplication for Abu Mahdhurah. There is no objection to saying "Barakallahu bik" (May Allah bless through you), as it is linguistically acceptable and appeared in some versions of the hadith. It carries the meaning of supplicating for blessings to be present with the person or to come about because of them.