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+.
Covenant for shadow priest in Shadowlands
While I don't want to officially call it the "BiS" covenant, currently Necrolord is by far the safest pick for Shadow Priests. It was already the dominant choice for multi-target damage, and it was really close to Kyrian without requiring you to completely change your playstyle. Recently Pelagos, which pretty much carried Kyrian for Spriest, was nerfed again, so it's likely that Necrolord is now the all-around best pick, but as this is very recent I don't feel comfortable officially declaring that on the rankings.