The One with the Proposal (Part 2) (Season 6, Episode 25)

The Plot: Chandler intends to propose to Monica, but her mightiest old flame — Tom Selleck's Richard — chooses to resurface at the worst possible time.

Monica and Chandler's relationship isn't as crazy as Ross and Rachel's; after they recognized they loved each other, it didn't take long for things to turn serious, and they weren't big on drama. Still, when Richard returns, it takes two episodes for things to settle down, resulting in one of the series' most poignant sequences, with the ever-sarcastic Chandler proposing to the one he loves in a straight-faced manner. Friends seldom become this serious, which is why the tonal shift works so well.

Link: https://www.rottentomatoes.com/tv/friends/s06/e25