I`m leary of third world countries for starters,but I`d probably choose England or Italy as my two top places.
Actually those two are on my Always Go list. There are some trips I will never pass up. Besides those two, France, Germany, Greece, Baltics, Switzerland.
A few places that I will always pass up : Isreal/Jordan/Egypt (been there more than once, no further interest), Mexico/Latin/South America, Africa and most of the cruise options.
One of the best trips I have ever done was when he put together an itinerary about WW2 history. It started in Normandy and followed the route that Americans took as they moved inland after the invasion. Covered a lot ground and small sites but some big ones too. Saw two of the concentration camps that were used near the end. Incredible experience.