What is the best covenant for Paladin Shadowlands?
BIS COVENANT FOR HOLY PALADIN IN SHADOWLANDS
While some Covenant abilities have been buffed over time for Paladin, it hasn't really affected Holy much. Divine Toll is going to be your go-to pick for all forms of content by a pretty significant margin.
BIS COVENANT FOR PROTECTION PALADIN IN SHADOWLANDS
Oh boy, where do I begin with Prot Pally. While they do have two strong picks in the way of Divine Toll and Ashen Hollow, it's honestly impossible to call either one the "BiS" covenant, as they're both so incredibly limited to their respective form of damage. Divine Toll is great on AoE, but it's TERRIBLE on ST. It's quite literally Avenger's Shield on a 1 minute cooldown when used on a Single Target. The fact that Blizzard still hasn't made it so Divine Toll fires all 5 shields regardless of target count boggles my mind. It's such a simple change that would completely change the viability of the covenant.
Also, while Ashen Hollow is the better choice for Single Target, it falls off quite a bit on multiple targets, especially when compared to Divine Toll. The state of covenant abilities for Prot Pally is extremely frustrating because of this. Personally, I'm going to be leveling two Prot Pallies because of how restrictive the abilities are, and if it's something you're interested in playing I'd recommend you do the same. While this is a really unfortunate situation, that's the reality of where things stand for Prot Pally at the moment.
BIS COVENANT FOR RETRIBUTION PALADIN IN SHADOWLANDS
Ret Pallies are pretty much in the same boat as Prot Pallies. You pick Kyrian for AoE and Venthyr for ST, and both are pretty awful at the opposite type of damage. If I REALLY had to suggest something for Ret, I would actually recommend Kyrian, as Judgement on a Single Target is less bad than Avenger's Shield is on a Single Target, so Divine Toll will likely be the more "flexible" option. Blizzard really should make some changes to Paladin covenants before Nathria comes out though, because currently the state of things is pretty bad.