The best thing this show ever did was hook Monica and Chandler up and have them date in secret, and that all culminated in this, the best episode of the series, which pits Monica and Chandler against Phoebe and Rachel (with Joey in the middle) in a battle for who can hold out on the secret the longest. It's a perfect 20 minutes of escalation, and Chandler's "I love you" is the cherry on top. They don't know we know they know we know!