All the covenants are fairly close in value, but for early on, Venthyr looks to be the best covenant for Unholy Deathknight with Kyrian taking over later in the expansion when more are unlocked.
Soulbinds and Conduits
The best Soulbind for Unholy Deathknight currently is Theotar.
This is most likely how your soulbind tree is going to want to look:
For potency conduits you want to go with Eternal Hunger and Convocation of the Dead.
Your endurance conduit is going to be Reinforced Shell. It increases the radius by 15% and the duration by 2 seconds of your Anti Magic zone ability; this is going to be the go-to choice for raiding. However, for Mythic+, you may find more success with the hardened bones conduit that makes it so while lichborne is active the damage you take is reduced by 6%.
Finesse conduits are a lot more open and really depend on what you need. If there are fights that require interrupts, then the spirit drain conduit (successfully interrupting an enemy with mind freeze grants 10 runic power) could end up being a very minor DPS increase.
For single target, deadliest coil is going to be the legendary you want to craft. Deadliest coil reduces the runic power cost of death coil by 10 and death coil increases the duration of dark transformation by 1 second. This legendary can be crafted for either the chest or the cloak slot.
For AoE, Frenzied monstrosity is going to be the strongest choice since you will spend your runic power on epidemic instead of death coil. Frenzied monstrosity causes your dark transformation to also increase the attack speed and damage of you and your monstrosity by 12%. It can be crafted for the head or hands slot.