Best covenant/soulbind for Single target: Kyrian Pelagos*
Best covenant/soulbind for Dungeons/AoE: Necrolords Emeni
*Kyrian Pelagos requires full Renown to really pull far ahead of the competing Covenants because it is one of the few soulbinds that has a dps node at both the start and the end of the soulbind tree. You can pick any of the covenants at the start of expansion and be competitive. Each of them has certain areas/fights where they pull ahead due to their conduit tree, their utility or the covenant ability. Then once you have enough renown you can swap to Kyrian Pelagos.
Soulbinds and Conduits
Currently, there are 2 legendaries that are competitive for shadow.
Eternal Call to the Void: This gives your spells a chance to spawn a void tentacle that will attack your target, dealing damage and generating insanity.
Currently, the Cabalist’s Hymnal trinket (especially if you have people in your party who are also using it) from the raid look very good.