Since I have recently been fortunate enough to see places I never thought I would get the opportunity to see, such as Normandy, Pointe du Hoc and Paris, and will visit Rome this April, I would have to say Thermopylae. Yes, definitely Thermopylae.
And then (since I forgot to list them, and in no particular order): Alaska, Versaille, Site of the Battle of Issus, Mount Vernon, Nasca Lines, Giza Pyramids, Australia, Greece, Antarctica (yes, Antarctica!).
Did you visit those places via river cruise? We took and Avalon cruise to Normandy, in April, and it was glorious We’ll never step foot on a large cruise ship again.
Number #1 on our list is Australia.