First set what your requirements are...
Are you all adults? Are you tall, small people? How big is your group?
For most average crowd days... if you want to 'pick' your spot from one of the desirable ones... expect to have to be there about an hour before showtime. These spots often are those along the walls of the garden areas in the hub. People pick a spot so they can get a clear view of the castle and have no one in front of them. They also provide a spot to sit or lean against while waiting.
As the crowds start to build, everyone will be sitting on the ground in the outlined areas in the hub. The castle show now is so projection heavy - you really want to see what is on the castle. I think the back half of the hub to the start of mainstreet are good areas. Basically back half of hub, along a wall with an unobstructed view of the castle is best IMO. All the areas will fill in leading up to the show.
But there is a trick if you are a small group and willing to wing it. Prior to the show... maybe 20mins or so (check better resources when this really happens)... CMs will go around and have everyone stand up and fill in the open spaces. When this happens, tons of available space opens up in what were areas that people had filled in 45mins earlier. So if you time it right, you can stroll right in and get a spot in the primo areas with almost no waiting! Just be walking on the paths when CMs start pushing everyone forward.. and you slip right into the open space. If you have good sized people, and are flexible... this works the best.