I asked the players on my team what pitch they fear most and they tell me its the change-up. They aren't afraid of a wicked fast or live fastball, they always feel that they can get around to hit it and can see it coming out of the pitchers hand- no deception involved- you get what you see- no magic.
The change-up on the other hand is dreaded by good hitters because of its extremely high deceptive value. You don't hear of good hitters waiting for the change-up to hit, no, they like the fastball to hit- always have and always will. The best pitch in my sons arsenal is his change-up. It gets hit less, creates more swinging strikes, and when it is rarely hit, it doesn't go anywhere far- almost alway a ground ball weakly hit to an infielder.
I have always felt that the best pitch a pitcher throws is the one the batter dreads. And for most players, it is the change-up or curveball- something other than the fastball.
Change-up/offspeed pitch is always best because of the deception value. It has been proven that any good batter can hit any good fastball located around the strike zone. Fastballs have no deception- you can see it coming and just swing and hit. Change-ups when thrown properly not only has the fastball appearance but has the compounded ability to have more late movement and timing issue for a batter to deal with.
How bad do hitters get fooled by it? Take this test for example- Try and throw for a first pitch a fastball or a change-up to a good hitter. Chances are you can float in a change-up right down the middle without good contact or any contact while almost always the fastball right down the middle for a first pitch will get creamed.
I don't know exactly what goes through a batters head when they are up to bat and swing foolishly at the change-up for a first pitch strike but it is so effective they can't fix it either. We did this approach with the best travel team in our area years ago. We pitched change-ups for the first pitch to batters on that team 75% of the time and every time they would swing out of their shoes and come up with nothing- they flat out could not tell it was coming slower and adjust to it. The coach was getting very ****ed off also- kept telling them not to swing on the first pitch, and so when they didn't swing they still got a strike.
A good change-up will make a fastball look better anyday than a good fastball making a change-up look better.