The One Where Everybody Finds Out (Season 5, Episode 14)

The plot of this episode is that everyone eventually discovers Monica and Chandler are secretly in a relationship. There are several Friends episodes, including the ones listed above, that die-hard fans will point to and exclaim, "That one!" As in, "Begin with that one." It finest exemplifies it." But, more often than not, they'll bring up this fifth-season highlight in which everyone (even Ross) discovers Monica and Chandler are dating.

The couple has kept it a secret since Ross' catastrophic London wedding at the end of season four, but everyone knows it won't last. Joey was the first to find out, followed by Rachel, and now Phoebe. Ross eventually discovers it near the end of the episode while trying to get to the Ugly Naked Guy's apartment while suffering from crippling anger issues.

Link: https://www.imdb.com/title/tt0583452