It could be argued all day long really. The CU is "the best pitch" in baseball in my opinion. I have studied the CU for several years and have made my own conclusions based off of the data. Like was said- calculating the numerical odds- taking both a mathematical approach and a results approach. It is true that everything is based off of a pitchers fastball (except for the few knukleballers and such).
The batter as such, times a pitchers arm motion as it relates to his velocity. Doing this gives him the greatest advantage for timing and hitting the fastball. Technically speaking, any pitch thrown at a lower velocity than the fastabll is an offspeed pitch. Mathematically, it is harder for the batter to pick up the velocity of a straighter pitch versus a "breaking" pitch. Many would thus think that the fastball would be the hardest to time. But this is not so, why? Because batters see more fastballs than any other pitch, they learn how to time the pitch. Mental triggers go off in the brain based first off of arm speed and then ball speed second. Breaking pitches lose their deception too quickly in general because as soon as the pitch begins to break, the batter can instantly calculate velocity and its projected plane and thus meet the ball better. The CU on the other hand, when thrown on the same plane as the fastball will not set off triggers to the mind until later in its flight over the other offspeed pitches of what its velocity actually is and its projected plane of trajectory. This is what makes it so deceptive. Batters who swing and miss actually think when they begin their swing that it is the fastball they are swinging at.
No ther pitch in baseball can mimick the fastball for a greater period of time during its flight. I have video showing that a batters timing motions are the same when he swings on a fastball or the CU. This tells me that batters must make an added adjustment to hit the CU- thus it is a harder pitch to hit because the batter must make more motions and calculations to hit it.
Breaking pitches are relatively easier to pick up what they are because of their early break away from the fastball trajectory. CU's traject on the same plane as the fastball the longest amount of time before dropping and fading in or out.