Thanks for the question. Personally, I think the curveball is a great strike out pitch. A lot of people will say the slider is the best, but I don’t like HS pitchers throwing the slider. I think the secret in throwing the curve is achieving top-side leverage on the ball and getting good elbow extension. If you would, please check out the elbow extension activity on his website. If possible, try a curveball grip with the exercise and monitor the results. The drill gets the arm in the proper pre-release position. Furthermore, try taking a baseball and coloring half the ball red. With the contrasting colors the ball’s spin will be easier to read. When doing these drills the pitcher should really focus on the rotation of the ball. An effective curveball will move from up to down or 12-6 in baseball lingo. Using the colored ball, the pitcher should receive instant feedback on their technique. Did I stay on top of the ball? Did I get maximum extension through release? These questions are answered by using the half-colored ball. When starting these drills, move the target closer and then slowly move to 60, 75, or 90 feet.
Also, work with the leverage on the front side. Are you getting good tilt with the lead arm? When practicing, try and exaggerate the front side tilt. Have you seen Francisco Rodriguez pitch for the LA Angels? He does a great job of getting leverage going to the plate and has one of the best curve balls in the game.
The two other pitches I would recommend would be the sinker and change-up. I like the sinker because it is thrown like a two-seam fastball, only the position of the thumb changes. Usually, sinker ball pitchers run the thumb along side the ball. Also, the sinker allows pitchers to maintain healthy arm action that is deceptive to hitters. The change up is another pitch that is easily mastered because of the simple grip and deceptive arm action. I hope this helps you, but remember to keep experimenting with all these pitches because you never know what off-speed pitch matches your throwing style. Heck, if your hands are big enough experiment with the split finger fastball. If you would, try the elbow extension drill and tell me how it works.