Of the choices you gave, January is definitely the least crowded. However, keep in mind that there would be things you could not do if the weather is too cold, i.e. rides you couldn't go on, water parks you couldn't go to.
Of the choices you gave, I'd pick September, preferably later in Sept. to give a chance to let summer weather cool off a bit.
Early December is indeed nice with all the Christmas decorations, etc. but we went first week of December this year and it was crowded.
If you went in September, it's likely you wouldn't have to wait much at all for anything and you'd be able to enjoy warm weather activities.
Have fun!