One great episode worth watching an entire series for

Okay, so I have a watch list like everyone else does. I prioritize movies over shows because I can go through more of them and feel complete instead of watching multiple long shows and trying to be invested in all of them. The X-Files is one of those shows that just feels like it's never going anywhere, so why the rush?

I see a comment about some X-Files episode, and the premise is blocked out. I see some of the responses, and it got me curious. It's the same with some other shows where I'll see an episode referenced and everyone is gushing over it. It makes me want to watch the whole show just so that I could be emotionally invested once I do get to these great episodes.

So here's my ask: What are some episodes that are worth watching the entire series up to that point for?

No spoilers, por favor!