Angkor Wat is truly one of the top wonders of the world. All your expectations will be met and raised up a notch. It is truly one of the greatest feats of mankind, a gigantic city build of stone gods. Other than temples, there is a carved up river bed up town which is pretty cool as well as many off beaten temples (but don’t think you are ever going on the off-beaten path, the city is pretty well maintained).
More Info: Wikipedia