Threshold. Three quarters of the way through a standard sci fi tech story the whole episode drops some acid. What's not to love?
I will grant that "Threshold," as goofy and nonsensical as it is, is actually more entertaining that plenty of dull, forgettable, middle-of-the-road type episodes. "Threshold" is jaw-droppingly insane--like a good episode of AMERICAN HORROR STORY.
Then again, I like the TNG ep where Barclay turns into a giant spider-monster, too.