In my opinion, this was the pinnacle of the series. Monica and Rachel versus Chandler and Joey competing for the girls' apartment with Ross as the judge. I think the comedic timing between these five actors was absolutely perfect during Ross's game show, and it's a scene I never get tired of. Unforunately, Pheobe's subplot, though important to a long story arc for this particular season, paled in comparison to the main plot.
Friends had a lot of entertaining episodes, and was consistently strong throughout its run, but "The One WIth the Embryos" stands out for its pace, writing, performances, originality and humor. (Actually Monica, although not my favorite cast member, was particularly strong in this episode.) If you ever had to introduce someone to this series, this episode would be the place to start.