Series 2, Episode 14
Joey is offended when Chandler makes fun of a gold bracelet he has bought for him to say thanks for his friendship. Ross continues his quest to land Rachel but she tells him to accept the fact it will never happen. And as Monica's parents come over to the apartment they bring with them an old video tape of her prom night... which when viewed by the gang reveals a lot more that Ross would have liked to have remembered.
At the conclusion of the final scene if you watch Monica in the background wipe a tear from her eye - that is exactly what I done myself as I was watching it for the first time. Brilliant.