I’m assuming you mean for Resto, yes?
Depends on what your focus is in keys. If you want something reasonably easy to play, which makes your already strong healing even stronger, Venthyr is a great choice. Chain Harvest is very nice, and easy to play.
If you’re main focus is raiding, and M+ is a side-gig, Necrolord is very much viable too.
If your main spec is elemental, Fae is decent for resto aswell.
If you want to do some very nice damage, then Kyrian is your go-to covenant. Vesper totem with the Kyrian legendary nets you more damage than Deeptremor, and none of the other legendaries even come close to those two as far as damage is concerned.
Mind you, all of the covenants are viable for Resto Shamans. Perhaps not for the highest of keys, but up to +25s or thereabouts, they all work. On the other hand, if your team lacks a Venthyr, then going Venthyr for SD and HoA, is more beneficial than not having a Venthyr but doing more damage.
Luckily, in 9.1.5, you won’t have to choose. You can try both, and decide which one you like best. Personally, I enjoyed the healing power of Venthyr, but… I like big damage numbers, and Kyrian delivered those numbers for me. =)