What is the best covenant for Priest Shadowlands?
BIS COVENANT FOR DISCIPLINE PRIEST IN SHADOWLANDS
Both Kyrian and Venthyr are equally as powerful for Discipline Priests, but it's generally for different types of content. For Raids, Venthyr is the better pick when used properly, but for Mythic+ you'll get more mileage out of the raw healing from Kyrian. While each of these abilities is relatively far ahead for its respective content type, neither one is "bad" in the opposite form of content, so you can comfortably pick either one.
BIS COVENANT FOR HOLY PRIEST IN SHADOWLANDS
It's hard to say what the best covenant is for Holy Priest, as most serious theorycrafting is focused on Discipline at the moment. That said, it's currently looking to be a relatively even toss-up between Kyrian and Necrolord for both Raids and Mythic+.