Bumping this thread a bit 'cause I tried a new place last time and it was awesome. Go figure, I also did a search instead of starting a new thread.
*pats self on back*
Anyway, a couple of people have mentioned the stairs in Canada.
Go to the top of the stairs and take a left. There's a little corner in there with a wall to lean on, and water fountains, to boot. If you're the first people there, you can lean on the wall, it's right by a speaker, and the whole thing is framed by trees--but not blocked in any way.
It's the first time I've seen Illuminations from an elevated view, and it was phenomenal, and slightly different.
Anyway, new spot for me, watched it twice from there, and it seems like a prime "secret spot".
Otherwise I'm big on World Showcase Plaza and San Angel Cantina.