American here...always thought the NL/Dutch accent was weirdly the most American-like accent? Which I assume is universally fairly easy to understand because of all the media content? I've been around Dutch natives before where it took surprisingly long before realizing they weren't from the US.

In my opinion the Scottish accent is the most difficult accent to understand.

"Introduced" the Scottish accent via Youtube to some Mexican co-workers who are fluent English speakers and they freaked out. Will never forget one saying "WTF is this? How can this possibly be real?". Guy generally thought it was some sort of fake prank way of speaking and couldn't grasp that people actually spoke like that.