Which mob scared or surprised you the most?

Maybe you're in a cave and a creeper blows up behind you or falls from high grounds in front of you to your demise (or maybe even destroys the ground you're standing on and leaves you in a pit of lava). Or maybe a bunch of cave spiders start attacking you as you start backing away from a mine shaft and walling the place. Or maybe an Enderman walks near water and starts teleporting like crazy. Or maybe lightning strikes and the four skeleton horsemen appear in front of you. Or maybe you're in a cave (or a cliff side of a ravine) and you thought you've cleared the place, but then a skeleton out of nowhere shoots you. Or you enter the Nether and you start hearing Ghasts.

There are a lot of moments where you wouldn't expect a mob to scare or surprise you. Which mob would you say scared or surprised you the most, and in what way?