That's a tough call! There are so many different places I would love to see.
I've always wanted to check out Machu Picchu in Peru. From the time I was a kid I remember seeing photos of it and thinking it was one of the most beautiful places I had ever seen. I can't decide if I would hike up there (a strenuous trip that takes about 3-4 days of intense hiking) or if I would take the train (3-5 hours without all the muscle strain and heavy breathing!).  Either way, I think it would be amazing to see in person.
I've also always wanted to visit Greece. It seems like there is so much to do and see there. Visiting ancient ruins and taking in all of the history would be fascinating. In the coastal areas, the water is such a gorgeous shade of blue, and the hillsides dotted with buildings that are either brightly colored or all white are absolutely breathtaking.
Photo by Adam Rifkin
Several more that are super high on my list are the pyramids in Egypt, Petra in Jordan, Pompeii and the Great Wall of China. Man, am I ever going to be jet lagged by the time I'm done! :)