Intellect > Haste/Mastery > Critical Strike > Versatility
Haste & Mastery are your best stats for Single Target. Haste usually pulls a little bit ahead especially if you're using Wracking Brilliance. Mastery pulls ahead a lot in every cleave/AoE scenario. Intellect gained a lot of value compared to Legion. That's why usually higher item-level pieces are better even with worse itemization. Rings are an exception.
I highly recommend everyone to sim their characters on Raidbots to find out their exact stat values. I made a little video for people with no knowledge about simming characters.
What do my stats do?
Intellect: Raw damage increase on everything.
Haste: Increases cast speed.
Mastery: Increases damage done to Agony, Corruption, Siphon Life, Phantom Singularity, Unstable Affliction & Seed of Corruption.
Critical Strike: Double damage on critical strikes.
Versatility: A weak damage multiplier & damage reduction.
The best way to calculate stat priorities for your character is to "sim" your characterFind out how to sim
Races are a lot less beneficial than they used to be however the best DPS ones are:
Horde: Troll > Goblin > Undead > Orc > Nightborne > Blood Elf
Alliance: Gnome > Dark Iron Dwarf > Human > Worgen > Void Elf > Dwarf
The enchants you use depend on your stat weights. Both Haste & Mastery enchants can be useful. You should sim yourself on Raidbots before enchanting your gear.
Gems are depending on your stat weights. Both Haste & Mastery gems can be useful. It is always best to sim yourself before socketing your gear. It is also recommended using one Intellect gem if your Intellect is simming better than both your Haste & Mastery.
Haste: Quick Owlseye
Mastery: Masterful Tidal Amethyst
Intellect: Kraken's Eye of Intellect
You should always use the following consumables to maximize your DPS:
Flask: Flask of Endless Fathoms
Potion: Battle Potion of Intellect
Vantus Runes: Vantus Rune: Uldir