What would it be?
I reason as follows.
Jab would be great and important in boxing, yet not what I would pick. Straight right or left is great, but I don't see a whole lot of knock outs off that either, and etc.
Overhand right is decent for power, but I would like something that perhaps have a shorter distance to travel.
You could go with a hook, you could throw with a power and speed and make it short and quick, but not sure if i would pick it either if I have to pick one.
There are uppercuts, but then it's not something that you would wanna open up with, unlike jabs, and it's rather rare to see one land clean enough to knock someone out.
So, I have really no idea.