The fourth episode of The Office is arguably the best of the entire first season, and a strong contender for the title of finest episode ever (the two-part series finale is stronger, though). Why? Because of a very poignant development in the platonic romance between Tim and Dawn, and also because of a song.
The premise of the show sees everyone disillusioned over the possible downsizing which might occur soon. In order to cheer them up, David organizes a Training Day that will boost his staff's morale. The situation gets out of hand (obviously) due to his inability to approach anything seriously, and things get more awkward when a rumor starts circulating about Dawn and Lee (Joel Beckett) calling off their wedding and Tim thinks it might be a good time to make his next move.
The entire mid-section, where various types of role play and trust exercises are suggested, is the single funniest moment in the series' history, perfectly accompanied by the somehow darker conclusion. Gareth has his best scene ever when asked about his ultimate fantasy ("Two lesbians. Sisters, probably. I'd just be watching."), but the crown must once again be given to Ricky Gervais for providing Brent with the definitive comic weapon: a ridiculous song he wrote himself. Forever associated with the show, it has become one of the comedian's most recognizable trademarks (along with another thing introduced in Season 2). All together now: free love on the freelove freeway...