The one that does the most damage. Depends on which you have access to so far. Also depends how many resources you want to spend per spell.

Slicing Winds is a great starter spell if you can hit 20k accuracy and has great synergy with surge + air imbued wand. If you can't hit 20k accuracy I'd go with ignite since it's a one time cast. You will only use one potion charge (diamond luck) per cast and prayer points once.

Quake can be the highest damage spell, but also has a chance to miss. Also depends on the mob you're fighting if you kill it off in one cast or if you need multiple.

Gust has the best damage per resource value as far as I know (Feel free to correct me if I'm wrong).

So yeah a lot of it depends on which mob you fight or which dungeon you want to idle. If you value your prayer points, diamond luck potions and runes you'll want to use Gust at 20k+ accuracy.