I would say that if you HAD to name the single most important fundamental of shooting pool, the stroke would be the #1 answer given by most people. It doesn't matter if you do everything else right, if you don't hit the cueball where you intend to, you're going to miss that shot (unless it's a really close shot). The other fundamentals, if not done using textbook form, can be overcome with a straight, smooth, repeatable stroke. As far as aiming goes, with enough practice this will come to you eventually. But it doesn't matter if your aim is PERFECT if you can't deliver the cueball to the correct point on the object ball because of a wobbly stroke.
I would say that as far as beginner to intermediate players go, more shots are missed by a bad stroke than by improper aiming.
Maniac