Tony Perez says: "See the ball, hit the ball"
Pickledad says ( :
): "To have good hand eye coordination, the "eyes" are pretty darn important (tracking).
So, I would say "tracking" if considering the mechanics, ... you are NOT going to hit that which you do not see, ... unless you are Luke Skywalker ("use the force"
.
Moreover though, in a more abstract approach, I really believe the most important part of an individual's swing, is their own mental prep in dealing with their own unique swing critical success factors, and "swing keys."
With all of the excellent training/instructors available out there today, most any kid can develop a pretty solid "lab" swing if they are willing to work, and the parents are willing to spend. Most professional hitters, and pro golfers, know that their "beautiful" and "professional" individualized technique can/will break down from time to time, and over time they learn why it happens to them personally. Therefore, they usually isolate their preparation thoughts to their own personally focused 2-3 swing keys - those things that will help guarantee that the entire "sweet swing" which they are capable of producing actually occurs - and it usually will, if those couple of weaknesses are overcome thru focus, especially at the "front end" of the swing. An internal focus on one's own performance capability & technique is priority one before moving on to the next most important mental prep part - having a plan for the pitcher/situation.
In conclusion, thinking physically/mechanics, it is tracking which can also be affected positively or negatively by the initial movements at the beginning of the swing (stepping/foot planting). But remember that hitting is 90% mental and the "Plan" is where it is at (internally & externally), given that the practice and muscle memory has been set in place for application & reproducability. Part of that plan is the focus on the hitters unique swing swing keys, and keeping it simple by focusing on just 1 or 2 things which will get you started correctly, and have other important memorized mechanical aspects like proper foot plant & tracking fall into place naturally.