I'm not too sure, maybe somewhere high; on top of Mount Fuji overlooking the rest of the world?
Or maybe in a nice hotel, glass windows all around, overlooking a busy city, lights everywhere, people bustling around.. it's a cool night but not too cold, and the room is warm and cozy. After the long day out of discovering new sights and having a good day, we step out, and share a kiss on the balcony. The noise of all the cars and people fading away, and just that good feeling of having found each other in this huge world filled with millions of people overwhelms us.. and it just feels right.
Personally, I think most romantic -time- is more important than most romantic place. Sharing a kiss after a proposal. Sharing a kiss after the loss of someone important to let the other know that you are there for them. Sharing a kiss after hearing some good news.. those kisses to me are the special ones- the ones that are the most romantic.