1. The Nightman Cometh (Season 4 - Episode 13)
Could there be another candidate? 'The Nightman Cometh' is an obscenely rewarding half hour for fans of the show, a brilliantly self-conscious exploration of each character's quirks and defects. The whole episode is based around the planning and performance of a musical written by Charlie, a tale regarding the previously-established figures of 'Nightman' and 'Dayman'. All the beloved tropes are here, culminating in a rapturous rendition of the 'Dayman' song. The fact that the entire musical concerns another beloved running gag - Charlie's relentlessly futile pursuit of the Waitress - ties everything together nicely, but this framework is an excuse for the gang to behave at their narcissistic worst. Paradoxically, the need for the characters to act badly actually shows off the cast's considerable acting talent. The musical is a comedic disaster from start to finish, featuring flubbed lines, horrendous musical numbers, and an amazingly self-aggrandising finale from Charlie (dressed as a human sun, of all things). It's absolutely no surprise that this episode was chosen for the Sunny live special - a unique gem in its own right - and it still stands out as the series' ultimate highpoint six seasons down the line.