I believe that the climax of the play comes in Act III, Scene 2, when the four lovers are at the height of their confusion. Both Demetrius and Lysander are pursuing Helena, and both Helena and Hermia are amazed at this. The guys go off to find a place to fight, and Hermia threatens Helena with a physical fight as well. They all run off and Puck and Oberon are left to trick them into the right areas to sort it out. The falling action begins in IV, 1 when Theseus and the other Athenians find the lovers in the woods and the triple marriage is set. My opinion is based on the main plot of the four lovers. If you focus on the Titania/Bottom subplot, the climax of that story comes in IV, 1, with Titania and Bottom at the height of their infatuation.
In literature and film, there are various stages which reveal aspects of the work being analysed in terms of its dramatic structure. This structure ensures that the story flows and reaches a satisfactory ending. The introduction or exposition when the characters, setting and basic ideas of the story are introduced helps the audience or the reader to identify with the characters and accept and understand the story. As the story progresses the rising action becomes apparent as conflicts develop and complications arise which build towards a climax. There is often tension and excitement and, in terms of A Midsummer Night's Dream, certainly confusion. The climax is the turning point in the story and the audience or reader ponders whether the main characters will succeed in their efforts or fail.
In A Midsummer Night's Dream, the comedic effects of Puck's mischievous magic and the resultant series of unlikely love choices, mean that things at this point could go terribly wrong. The fate of the main characters lies in Puck's hands and he has the power to correct the effects of his love potion or to watch the lovers make mistakes and continue in their bewilderment. Towards the end of Act III, scene ii Helena refers to Hermia as "little but fierce" (325) and Hermia is incredulous at the events that are unfolding and Demetrius and Lysander have resolved to fight for Helena. Therefore, this is a turning point or climax and only intervention by Puck can resolve the conflict.
The falling action will subsequently reveal how the conflict will be resolved and the resolution will ensure that things return to normal or, in A Midsummer Night's Dream, love will prevail and will resolve all the issues ensuring that all the characters are content with the result.