If I could take any pitcher to retire any batter at any point in the history of baseball, I might choose Aroldis Chapman at present. If I had to choose the worst pitcher in MLB in late May, I might have chosen Aroldis Chapman. Chapman didn't allow a run in his first dozen innings of the year, but in his subsequent four appearances, he allowed ten runs while recording four total outs. Over that stretch he walked 12 of the 19 batters he faced, bringing his seasonal line to 20 walks in 13 innings. Chapman was sent to the minors, but at some point he figured things out. Since being recalled in late June, Chapman has tallied 41 strikeouts compared to eight walks in 23.1 innings. I compared the PITCHf/x data of Bad Chapman (April/May) to Good Chapman (June-Present).
Here's a representative pitch from April:
And one from July: