1. I like to watch Spectro sitting on the curb facing down the road to Liberty Square. The parade makes a turn there, and if you look right you see the fronts of the floats with the castle behind it. Look left and you see the backs of the floats (which are really neat - how many of you have noticed the Cheshire Cat before??)
2. If there are two showings, the second one is almost always less attended. I've walked up to front row seats in Liberty Square just as the lights went out for the parade.

(I can't always cut it that close, but usually 10-15 minutes is plenty early).
3. If you've seen the parade up close, try watching it from far away. I like the Plaza Pavillion (I usually try to get Wishes seats there, those also work for Spectro). There are so many details you can't see close up! Musical notes coming out of the piano - fireworks over the last several floats. Beautiful stuff!
So, all that said, which is my favorite? It really depends on the day - if it's been a busy full day and I'm with a friend I haven't seen in forever, I'll take option 1 or 3. Lots of good talking/eating/resting time built in. If I've got to have one last ride on the Haunted Mansion, I take option 2.