I've been to 9 of these (except Grand Canyon - I can do that when I m old).
Apart from Eiffel Tower - I agree, they are worth crowds - and sweating like a mad man (Taj Mahal in August)
Prague's old town? There are equally nice "old towns" elsewhere in Europe.
I didnt see any crowds on either side of Vic Falls (especially Zim side in 2009)