You want to have as many secondaries as possible, so in most cases, item level will be the most important. However, if you have the option to choose, always go for Mastery.
Single Target Priority:
Strength increases your attack power, which is what determines how much damage your abilities do, so this should be on all of your items.
Mastery, also called Dreadblade, for Unholy increases the damage done by your minions and shadow damage abilities. Since shadow damage and minion damage makes up the most your total damage, this should be your second priority when looking at upgrades.
Critical Strike increases your chance to critically hit with all spells and abilities including your pets. Hitting a critical hit means your spell/ability will hit for twice the normal amount.
Versatility increases all your damage and healing done, whilst also reducing the damage you take.
Haste increases your rune regeneration and increases your attack speed.
The best way to calculate stat priorities for your character is to "sim" your characterFind out how to sim
For Horde, the largest DPS boost comes from the Orc Racial Blood fury. However, Blood Elf is not that far behind, and it provides utility through Arcane Torrent, which likely won't have a use in raids but is often used in Mythic+.
For Alliance, the best race is Mechagnome which gives you Combat analysis (a stacking main stat buff). But, if you’re more focused on Mythic+, it might be worth taking a look at Night Elf as it can be very useful to get out of combat or for doing trash skips using Shadowmeld.
Weapon: The best weapon enchant is the runeforging enchant, rune of the fallen crusader. It has a chance to heal you for 6% of your maximum health and it increases your strength by 15% for 15 seconds.
Chest: The chest enchant you want to use is Eternal skirmish. It increases your strength by 20 and gives your auto attacks a chance to deal additional shadow damage.
Ring: The ring enchant will be the tenet of mastery, increasing your mastery by 16.
Gloves: The glove enchants you want to use is called eternal strength, increasing your strength by 15.
Cloak: The cloak enchant will be the fortified speed increasing your stamina by 20 and your speed by 30
You would want to use a Masterful jewel cluster in all slots. Each Masterful Jewel Cluster increases your Mastery by 16.
Flask is going to be the Spectral Flask of Power.
Health Potion: The new health potion for shadowlands will be the Spiritual Healing Potion and of course the warlock healthstones since you get one use of each per fight.
Armor Kit: Heavy Desolate Armor Kit for your chest.