I've been to Niagara Falls more times than I can count, and it is ALWAYS simply incredible. I've stood by the falls for hours, until soaked by the spray. I've also been there when the smaller section (The American falls) was frozen.
The thermal pools of Hierapolis: now think of taking a bath in an ancient pool, into sparkling water sitting on a piece of a fallen column that was standing there in times you studied in your history books. Just amazing.