I'm betting on "Defying Gravity" too. Both gals get to sing, it has a special effect that will sell the show's magic, and frankly, it's generally considered the most exciting moment in the show. I don't think "For Good" will have power out of context, since it requires the experience of the first act relationship. Remember, Idina did "Gravity" alone on Letterman, complete with the elevation, and it worked well. Add La Chenoweth, and the rabble breaking in -- you've got a moment that will result in a lot of tix being sold.
"I'm a comedian, but in my spare time, things bother me." Garry Shandling