Not that I'm actually taking this thread seriously, but the question asked about "the biggest" theater city? Unless they meant by square mileage, there's only one answer.
Finish each day and be done with it. You have done what you could. Some blunders and absurdities no doubt crept in; forget them as soon as you can. Tomorrow is a new day; begin it well and serenely and with too high a spirit to be enbered with your old nonsense. ~ Emerson
That depends on how you define New York City. Since Manhattan is the only borough designated as New York, New York, then the largest theater city would be Chicago (since Chicago is larger than Manhattan, and LA isn't known for legit theater). If you are including the boroughs of New York, which collectively make up what is called New York City, then yes, New York is the largest. But semantics aside, I'm pretty sure the original poster meant in terms of the amount of theater and the diversity of theater that is seen.