In an art where the whole point is to disrupt, throw off and in some cases - destroy - a hitter's timing, the changeup remains the most 'powerful' type of pitch one can have in his repertoire. Not only is there no other combination in baseball more effective at messing up a hitter's timing, it also puts significantly less stress on the arm than a curveball, slider or splitter. And although it has more to do with arm motion than anything else, it can also feature equally as nasty of movement as the other mentioned pitches.
From 1997 to around 2001, I don't think there was really any doubt as to who this title belonged to. The man's changeup was so incredible he could punch you out on three straight without even showing you the 97 MPH heater. However, over the last few years new strikeout pitchers have emerged, using and mastering the changeup as their bread and butter for domination. Unlike Greg Maddux, who probably had the best of the early 90s, the best change-ups of today feature incredible movement and cause hitters to swing and miss... frequently.
You've got Jason Schmidt and Eric Gagne, who will regularly throw 96-98 MPH before going down a notch and delivering changeups with splitter-like motion in the 84-88 MPH range. You've got the master of the circle change, Johan Santana, who throws in the low 90s before droping one 14-18 MPH slower, with "parachute" movement. And of course, El Rey, who still throws in the 90s with his change featuring very rare screwball-action movement.
Honestly, I don't have any idea which is the nastiest. I would love to just sit down and watch strikeout clips of each of them throwing their best changeups one after the other. Perhaps there are others who already have their mind made up on this debate...