What is the most common translation of ASAP (As Soon As Possible) in Spanish?
I have seen:
- cuanto antes
- cuanto antes posible
- lo más pronto posible
- cuanto antes, mejor
Are these all common and interchangeable, or is one used most often?
I would not say cuanto antes, mejor, because that's more like "the sooner, the better", which doesn't have the sense of urgency of ASAP. "The sonner, the better" is probably true for any task that you are told to do ("if it can be soon, that would be better"), but ASAP means that it has to be really soon (urgency).
Also, cuanto antes posible sounds incorrect to me, the correct form being lo antes posible, as in Sergio's answer. I would also add:
The correct one will be given by how urgent the task is and how formal the petition is.
All the forms are common and interchangeable, but the fourth one sounds a tiny bit less formal. Personally, I use 'lo antes posible'.