Allow me to start off by saying that A Midsummer Night’s Dream is one of the best ensemble shows in Shakespeare’s canon. Questions about the most “important” character are a bit strange, since the power, beauty, complexity and comedy of the show derives from how different characters interact with each other. Choosing a most important character in this play is akin to choosing the most important spoke in a wheel.
With that said, here are a couple answers:
The character with the most lines is Bottom, who has about 261 (depending on the version of the play). That is a very small number of lines for a leading character in a Shakespeare play to have; in fact, it may be the fewest of any play he wrote, underscoring how good of an ensemble show A Midsummer Night’s Dream is. That’s a comfortable number of lines one might expect a meaty secondary character to have in other plays.
Bottom is certainly an important character, and supplies a lot of the comedy in the show. But if the most important character is the one who moves the most plot and brings about the most change, the play could hardly function without Oberon and Puck, and Puck more specifically.
Puck is the one who transforms Bottom into an ass, and administers the love potion to the lovers. It is through Puck’s mishaps that much of the comedy and plot of the play come about. Puck may not have much of a motive or internal logic, but they are a character of action and accidents, a force of nature that moves the play along to its eventual end, where they give the epilogue.