So it depends on what your focus is.
If you really want to see the projections and get a good view of things flying over the castle, it's best to be between the Partners statue and the Castle. You have to be careful though because there are lots of trees that could obstruct your view if you are off to the side. Unfortunately the spots in the center get taken up hours in advance, so you have to wait a while to be there. Alternately you can get on the other side of the Hub, between partners statue and Y-intersection of the road. That's where I saw it, and it was a good view, but definitely projections were obstructed and it was far off to see Buzz and the Up house. I do recommend being in the Hub, because they do something special with spotlights, which if you are on Main Street, you don't see as much (or at all).
Main Street also provides a unique view, as there are projections along the walls. I have yet to see the new show there, but plan to try. It's pretty far from the castle though so you really aren't going to be able to see the projections on the castle itself.
Then there are the Rivers of America (if you see the first showing of Fantasmic), or Small World. Not ideal for Together Forever though.