Not sure if I missed this somewhere in the thread, but shouldn't a lot of it depend on where you have/have not been in the past?
For instance, growing up in the states, I had been to many cities in the americas and europe. However, the first time I landed in Hong Kong and viewed the harbor from my room at the IC, that was life changing for me. I have been back many times, and now stay at the FS, and even then the IC wasn't the best hotel, but having never been to asia, it was like something I could not have imagined. Not just the hotel/view but the energy in the city was amazing. My grandparents who grew up in Europe felt the same way when they went to Shanghai/Beijing
I have friends who have felt that way in Jerusalem, SE asia and death valley.
If I were looking for a life changing experience, I would think about where is the farthest place from my every day existence. Which is also why I plan on going to Africa next year for my 30th...