When Shakespeare wrote his best comedies, he usually has the main and most important character speak almost always in verse. In A Midsummer Night's Dream, that character would be Puck, the mischievous and naughty assistant to the fairy king Oberon. Another way you can tell Puck is the most important character, is he changes the plot line a lot.
Puck is the main character, without him, there would be no storyline, or conflict. An example of that would be if Puck had put the magic flowers juice on Demetrius's eyes instead of Lysander, Lysander would not have fallen for Helena, like he did. If the Magic juice had gone to Demetrius, there would not have been much conflict in that scene, so it would be dull