Whats most important?

If you discount the enjoyment, and want to put the technicalities in order
of preference, ( somewhat dependent on where you are fishing, what with, and
what for),then mine would be;

Catching fish. Preferably large or difficult fish.

This may only be consistently achieved by the use of;

Stealth.
Reading the water.
Stealth.
Presentation.
Stealth.
Choice of fly, to suit either the hatch, or conditions.
Stealth.

This is also easier if you know the water well.

It also helps considerably if somebody has not just disturbed the water in
some way.

All the rest may or may not be useful, but these are essentials.

TL
MC