Early October was really nice last year. Was a little bit drizzly but it was really the perfect temperature (and no sunscreen needed!). Parks were empty and holiday overlays were in place. Was able to do every single ride in one day at DL and favorites twice.
Unfortunately, I prefer the Christmas overlays. The Halloween ones are fun and everything but the Xmas decorations are just magical IMO.
If you have a peticular season you want to exploit it might help narrow down a time better.
If not, just keep in mind slow seasons are when rides are down for refurbishment so if that is a big deal let us know. Also, as Hydro pointed out, Carsland is going to be a big factor in deciding a trip next year.