As it stands, Kyrian, Necrolord, and Venthyr are all extremely close for Elemental in terms of balance. Currently it appears that Necrolord is SLIGHTLY ahead, albeit not by enough to warrant a BiS ranking.
bis Covenant for enhancement shaman in Shadowlands
While they've buffed some of the underperforming covenant abilities slightly for Enhancement, the best options are still Kyrian and Venthyr, with Venthyr having enough of a lead to warrant a BiS ranking.
bis Covenant for restoration shaman in Shadowlands
Resto shamans remain completely unchanged, as their balance has been consistently good throughout the Shadowlands Beta. As long as you aren't picking Night Fae you really can't go wrong as a Resto Shaman, though many people say Necrolord feels much better in raids than it does in Mythic+. As far as throughput and mana efficiency go, all three abilities are almost dead even, so I'd recommend picking the one that fits your playstyle more.