Your stat priority for any content, let it be ST, AoE, Cleave, etc., 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 tics your DoTs deal per cast and makes your GCD and casts shorter.
Mastery - increases damage done by most of your primary abilities. Mastery
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 characterFind out how to sim
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.
Weapon - Sophic Devotion
Chest - Waking Stats
Legs - Frozen Spellthread
Rings - Devotion of Haste
Cloak - Homebound Speed
Wrist - Devotion of Speed
Feet - Plainsrunner’s Breeze
Don’t forget to make 2 extra sockets in your necklace with Medallion.
Skillful Illimited Diamond - 1 unique gem
Keen Ysemerald - Everywhere else
Flask - Static or Isolation if you can manage a high uptime of those. More realistically you are going to use Elemental Chaos.
Food - Banquet
Potion - Elemental
Weapon Rune - Howling
Rune - Draconic
Health Potion - Refreshing Healing Potion