How do you judge the quality of a pitch?
Not by velocity alone. Yankees right-hander Nathan Eovaldi owns the second-fastest fastball among starting pitchers this year, yet his fastball wouldn't make anyone's top-five list. And Royals starter Yordano Ventura has been struggling quite a lot for a guy with easy 95 mph cheese.
Not by movement alone, either. Rangers lefty Martin Perez has a sinker in the top five in horizontal movement, Mariners veteran Hisashi Iwakuma has a show-me curve he sometimes breaks out that has top-10 horizontal movement, and Joe Ross' changeup has top-five horizontal movement for a right-handed change (though he rarely throws it). These all might be OK pitches, but they aren't league-leading pitches.
The best researchers of our time are throwing complicated stats -- spin rates and velocity differentials, among others -- into the machine to try to better their ratings of pitches. While their work continues, there's an easy way to look at pitches right now: results.
Not results on balls in play, which are subject to the whims of the odd glove, the unraked infield or the weird bounce. Let's instead look at the pitches that best get whiffs and also grounders. One gets you closer to a strikeout, and the other limits damage on balls in play. My research has suggested that whiffs are about twice as important as grounders, so the basic formula for pitch quality here is two times relative whiff quality plus one time relative grounder quality. Using Z-scores, we can sum these up, while limiting our finalists to pitches thrown at least 150 times so far this year.