For dialogue, the longest laugh I can remember (which erupted into rippling sustained applause) was in the Kennedy Center MAME, when Mother Burnside made her entrance looking so distressed over her son's engagement to Mame, and said "What do you mean I don't look happy, Beauregard?" The timing was exquisite, and you could feel the power of such a big laugh on an audience - the laugh at her entrance seemed to energize the whole scene which followed. (Mary Stout played Mother Burnside, Eric Schaeffer was the director.)