The best pitch in this game is pretty random and based on your pitching pattern, it's basically the highest confidence pitch you can locate, in a place you haven't thrown it recently.
And that makes sense to the average gamer, but this is something that I really dislike about this game.
You should never, ever, ever, throw a 12-6 curve high in the zone. And if you ever did it more often than the ball slipping off your thumb early, you should be punished by watching the ball sail about 500 feet.
That's Wainwright, hooks vs RHB, I use him because he throws tons of deuces with great command. If he could throw high curveballs for strikes without getting torched, he would. The high release makes the grip very obvious to hitters, and the high trajectory makes it look like a big fat beach ball.. High breaking pitches of any kind are like tight spinning little paychecks to good hitters
The game doesn't expect casual players to know that, so it really doesn't punish you for it.
Ignoring real life, I find the best pitches are inside fastballs off the plate(4S or 2S) belt high sliders that break off the plate(low corner away breaking slider should be more effective, but it's not), and high fastballs (4S, running)on either corner.