I proposed to my fiancée last Sept. 2 in the Cinderella Rose Garden during the Fantasy in the Sky Fireworks. It is a very nice location, especially during a beautiful evening, though I went through Disney Engagement Services so the rose garden was roped off for our privacy. It would still make an excellent spot though, even if it isn't roped off for you alone...., surrounded by fully bloomed roses of varying colors, Cinderella Castle sitting majestically as the perfect backdrop, and the fireworks going off over head to the sounds of Disney music.....
Good luck whatever you choose.
Mirth