Chandler and Monica develop a romance that's arguably the best - and most healthy - relationship of the show. It doesn't rival the drama of Ross and Rachel, but it started organically and their love remained constant throughout the rest of the series.
While there aren't too many "big emotional arcs" of Season 6 aside from Chandler's proposal, this particular stretch of episodes is just some really well-crafted comedy.
Producers [Kauffman, Crane, and Bright] have managed to make this show, now in its sixth season, into a Rubik's Cube of comedy: They can twist and turn its characters in endless combinations.