Which pitch is the hardest for the catcher to block?

Fastball, curveball, slider, changeup, split, knuckleball. Which one of these pitches is the hardest to block? You might think it would be the pitch that has the most spin. I mean, isn't it logical to think that pitch would be the hardest to keep relatively close to the catcher when it hits the dirt. Well, it is true that the curveball (top spin) and the slider will move opposite of the ball flight once they hit the dirt. However, in the grand scheme of things, this doesn't quite make the list of level of difficulty for catchers who are blocking pitches.

The fastball is undeniably the hardest pitch for a catcher to block. Here's why.

When a catcher calls for an offspeed pitch he thinks this ball might be in the dirt. He is mentally prepared to block the ball. Regardless of the spin that the ball has, the catcher is able to handle it because he is anticipating it. However, when the catcher puts down the #1 he fully expects the ball to be thrown in the air. This expectation makes the fastball the hardest pitch to block.

The ball gets on the catcher the quickest and often times without the proper anticipation.