This is a toughy because Brandon Morrow's slider is absolutely filthy. Morrow threw his slider over 712 sliders in 2011 and gets almost 30 inches of vertical drop, including gravity. Horizontally, the pitch comes in at about 4.5 inches, and tops out in the high-80's in terms of velocity.
But, Morrow's slider isn't quite as filthy as Sergio Santos'.
Santos acquired a wSL of 9.0 despite only throwing the pitch 317 times. While his 32% reliance on the slider is somewhat worrisome, it's absolutely unhittable. Batters only put 8% of his sliders in play and whiff an astonishing 34% of the time. At 86mph, Santos gets a full 7.39 inches of movement on the horizontal plane and -34.6 inches on the vertical plane (again, gravity included.)
Morrow is right behind Santos on both the fastball and the slider, and extra points must be given because he starts games and faces the order a few times through, but based on just pure stuff, Santos has to win.