Best mage spec in WoW: Shadowlands 9.1 is pretty clear cut. Mages are in a fantastic spot right now, as all three of their specs: Arcane, Frost, and Fire are all viable in their own right.
Best Mage Spec in WoW: Shadowlands 9.1
If you want to play completely optimized and pump out the most damage your class has to offer, then Arcane is the way to go.
Currently, Arcane is slightly outperforming Frost in terms of raw numbers. Frost is right behind Arcane, with Fire a bit under their bluer brothers, but still able to dish out nice damage.
If you are not concerned with full optimization, any of the three specs are a great choice. It really does come down to preference across the three specs in terms of gameplay that you want to seek out.
A challenging but rewarding spec with Arcane, one of the very few specs left in the game where mana management is even a concept. Frost is pretty straightforward but streamlined and fun to play, and Fire has lovely burst windows, even if the cooldowns overlap and become a little awkward at times.
Mage is in an excellent spot, and all three specs are viable choices to bring into all forms of content, but Arcane is the top dog of the three as it stands in 9.1.