Always try to simulate your character using raidbots or the simulationcraft program to get personalized results.
Single target / Multi target:
Intellect: Increases the magnitude of your spells.
Haste: Increases spell casting and dot tick speed, reduces the cooldown of some spells and lowers the global cooldown.
Mastery: Increases your damage on the target for each Damage Over Time effect that you have active on the target. During Void Form all targets receive the maximum effect.
Versatility: Increases damage and healing done and reduces damage taken.
The best way to calculate stat priorities for your character is to "sim" your characterFind out how to sim
The differences between the races are very small so you should play whatever race you like the most. Except for Vulpera. Don’t play Vulpera.
Now, if you just want to play the best, then:
Goblin. Rocket jump provides instant on-demand mobility. It also provides the highest dps gain for horde through the 1% haste bonus.
Gnome/Void Elf. Void elf is slightly ahead on pure single target, gnome is slightly ahead in cleave/AoE.
Chest: Eternal Insight
Bracers: Eternal Intellect
Rings: Tenet of Haste
Haste: Quick Jewel Cluster
Weapon: Shadowcore Oil
Chest: Heavy Desolate Armor Kit
Flask: Spectral Flask of Power
Damage potion: Potion of Spectral Intellect