Main Street, particularly near the circle that houses the Partners statue. You want to have a full view of the castle to see Tink and Dumbo fly. On a not so busy day, we try to get a spot about 1 1/2 hours prior, but during busier times 2 hours.
ETA: We were there a little less than a week ago and only got a partial fireworks show without Tink and Dumbo and with a reduced amount of fireworks. I think the wind was a little stronger than was preferable for the full show. So that is always a possibility as well.