For all three I think its safe to say that Necro is the best answer. For just m+ I think it would be NF now. Wild Hunt + First strike is amazing and then you add on Fae Transfusion its going to be hard to beat. It feels really good to finish off your burst with a fae transfusion and then top people off.
Night Fae is the best choice. It’s competitive if not better than necro in M+ and is currently the BIS covenant in PvP because of Lava Burst nerfs. Night Fae is also becoming BIS for resto the more gear we get. And Night Fae might be slightly behind venth for enh but not by as much as necro. Furthermore, Night Fae has better soulbinds for enh and resto, while still being competitive for Ele.
If you plan on playing ele and resto ONLY then necro is probably the play for you for raiding. If you are going to do more M+ and PvP AND want to play enh then the best choice is Nightfae.
Necro is still considered BiS for ele (and resto if that matters to you), but with the 9.0.5 buffs, you can do fine with either Night Fae or Venthyr. Night Fae offers the strongest soulbinds for shaman, as well as an extremely useful blink ability. The downside is that fae transfusion is one of the more fiddly abilities to get the most use out of. Vesper totem is also very fiddly, but I don’t think anyone would recommend going Kyrian.
For PvP, Night Fae gives you the Dreamweaver soulbind, which gives you a cheat death mechanic. However, for ele, both Night Fae’s fae transfusion and Venthyr’s chain harvest are susceptible to kicks, which means you have to bait those out before casting. Fae transfusion is also an AoE that can be moved out of, so it requires its target to be stunned, so fae transfusion can be more tricky to get the most use out of (and probably requires coordination). However, when you can get it off, it trucks!
I’ve been Night Fae since I chose my covenant, and play all 3 shaman specs (though I main enh), and I’ve never really wished I had chosen a different covenant. The 9.0.5 buffs were really just gravy.
Lucky for you the shaman covenants for both Resto and Ele are pretty balanced… you will not be gimping yourself going to any of them. People generally prefer Necro, as it makes our playstyle a little more flexible, but HPS-wise, utility-wise, they are all pretty even.
I don’t know about enhance though… I hear necro is bad for it.