Single Target: Agility >> Mastery >= Critical Strike = Haste > Versatility
Mythic+: Agility >> Critical Strike >= Mastery >= Haste = Versatility
Critical Strike and Mastery hold a core part of Assassinations damage. In Dragonflight, a large portion of Assassinations damage is affected by mastery, which makes it a very efficient stat. Critical Strike has a lot of support in the class tree talents, including Seal Fate, Deadly Precision, and Lethality. These all provide a large amount of benefits rotationally. Haste affects damage over time tick rates and adds a small amount of passive energy regen. Haste is also quite good but gets overpowered by both Critical Stike and Mastery. Versatility rounds out the bottom as a generic passive damage increase with no interaction. Having a little more Versatility than normal can be good in high mythic+ keystone (I would start worrying about it around +23/+24 traditionally), where the damage scaling on some abilities becomes a serious issue for your survivability.
The best way to calculate stat priorities for your character is to "sim" your character. Method Raider Jaerv has written a series of Metafy articles to help!How to Sim in World of Warcraft
For Horde I recommend Orc, as their racial ability Blood Fury pairs perfectly with our 2 minute cooldown Deathmark, and can be macro’d together for an easy burst. Similarly for Alliance, I recommend Dark Iron Dwarf for their racial ability.
Weapons - Enchant Weapon - Sophic Devotion
Chest - Enchant Chest - Waking Stats
Boots - Enchant Boots - Watcher’s Loam
Mastery and Critical Strike: Sensei’s Neltharite
Weapons - Hissing Rune
Phial - Iced Phial of Corrupting Rage
Leg Armor - Fierce Armor Kit