OK, I don't know the best month but I did go last Christmas, although crowded it was AWESOME The weather was perfect, warm in the day, cooler at night, no jacket needed for me anyways and I am always cold. Now this year I am going in Sept. I figure school is open for I think every state so the crouds should be low this time but then again I could be wrong. I think no matter when you go you'll have a great time and not even be worried about the crowds or other things.
I would have to say May because that is the only time I have been. Actaully what ever month I could go would be my favorite!! I would love to see DL at Halloween and Christmas. If I went at that time, I am sure that my answer would change.
We love mid November the Christmas decorations are up the weather last year was awesome and the crowds were there but you know what it is Disneyland, you can deal with the crowds!