My choice is based on ‘nostalgia’ value and I only chose those man-made structures which are no longer in existence. In this regard, the National Stadium only just made it because it is being demolished.
And here is my list, not in order of preference.
1) National Library at Stamford Road
2) National Theatre
3) Van Kleef Aquarium
4) Haw Par Villa
5) The Merdeka Bridge of Nicoll Highway
6) National Stadium
7) Nantah Arch at the former Upper Jurong Road
8) Special mention - An additional ‘wonder’ applicable only to ACS old boys: Anglo Chinese School Clock Tower