Best covenant/soulbind for Single target: Night Fae Niya
Best covenant/soulbind for Dungeons/Raids overall: Night Fae Niya
All the covenants are relatively close to each other in terms of DPS. If you like the aesthetics or the utility of a certain covenant don’t hesitate to pick them.
Soulbinds and Conduits
The 2nd best legendary is Talbadar's Stratagem: This increases the damage of your Mind Blasts by 60% if Vampiric Touch, Shadow Word: Pain and Devouring Plague are on the target. This legendary is slightly weaker than Shadowflame Prism on single target and it has no AOE/cleave value. If you use this legendary make sure to use Void Torrent instead of Mindbender.
As an outsider I would also like to mention the Twins of the Sun Priestess legendary. This is not a DPS increase for the priest, but can be useful to overcome certain phases where extra raid DPS is required. A good example of this is Sludgefist where you overlap the Power Infusions with the 100% damage taken increase on the boss.
The Cabalist's Hymnal trinket is good especially if you have people in your party who are also using it.
Another good trinket is the Soulletting Ruby from Theater of Pain.