For me, it's The Witch. I found the two parter WttH/The Harvest a bit cringeworthy in parts, plus the vampires were boring. The Witch was the episode that made me sit up and think there was more to this show than first seemed.

The Witch is just a great little episode and rather than repeat why I think so, I'll just link to my review. I was a bit rude about cheerleading. Sorry.