The one that immediately came to mind was "The Ski Lodge" episode of Frasier. Peri Gilpin isn't in it much, but the other regulars and guest stars James Patrick Stewart and Cathy Lamontange are all completely integrated and hysterical. Every scene has every character.
It's already been mentioned but no one did ensemble episodes like "Friends" did. It wasn't a perfect show in every aspect, but the show soared when the cast was all together.
They did it year after year with Thanksgiving episodes, mixing comedy and drama (Ross and Rachel's break up with the other four hiding out in the bedroom), "event" episodes (TOW Monica's Thunder, post Monica and Chandler engagement), episodes about "nothing" (TOW No One's Ready). Even when the show started to show it's age the ensemble heavy episodes were worthy (TOW the Lottery, TOW the Cake).