Your stat priority for any content (Single Target, AoE, Cleave) is going to be:
Haste >= Mastery > Crit >= Vers
There are no hard caps or anything as a affliction warlock, so you should go for items with Haste/Mastery.
Intellect - Increases the damage and other values on every spell.
Haste - Increases the amount of ticks your DoTs deal per cast and makes your GCD and casts shorter.
Mastery (Mastery) - Increases damage done by most of your primary abilities.
Crit - Grants a chance to deal double damage on all spells.
Versatility - Flat increase to your damage, heal and reduce damage taken.
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
The best races for Affliction warlocks will be Orcs or Trolls for Horde players and Human or Mechagnome for Alliance players. Also worth noting is that the Dwarf racial can be very strong for M+ utility-wise.
Horde: Orc or Troll
Alliance: Human or Mechagnome
Weapon - enchant weapon - Sophic Devotion
Head - Incandescent Essence
Chest - Waking Stats
Legs - Frozen Spellthread
Rings - Enchant Ring - Devotion of Haste
Cloak - Enchant Cloak - Homebound Speed
Waist - Shadowed Belt Clasp
Don’t forget to make 2 extra sockets in your necklace with Medallion.
Fierce Illimited Diamond - 1 unique gem
Keen Ysemerald - Everywhere else
Phial - phial of static empowerment or phial of charged isolation if you can manage a high uptime of those. More realistically you are going to use Iced Phial of Corrupting Rage due to the phial not having any requirements to gain its benefit, though there are some scenarios where the damage increase combined with the reduction in damage taken from using Phial of Tepid Versatility should be considered especially in a progression raid boss setting or high Mythic+.
Food - grand banquet of the kaluak
Potion - elemental potion of ultimate power
Weapon Rune - Howling Rune
Rune - Draconic Augment Rune
Health Potion - Dreamwalker's Healing Potion