What do you think is the most important aspect in fly fishing?

Your at a gold medal, medium sized stream, trout are 10" to 20"
and they're all over the place, but won't take anything. The area is well
fished, the fish are spooky but also used to fisherman being around so, they
don't run and hide. No hatch, no fish rising.



Casting?- putting the fly exactly where it needs to be.
Fly selection?- match what they're eating!
Presentation?- drag free, correct depth.
Stealth?- don't let them know you there.

I realize all of these aspects are very important to a successful trip
but only pick one!.
I've only been fly fishing a few years but,
my guess would be presentation.
Just curious about what you guys think.

Thanks,
Dave Mc