I think any building which succeeded in reflecting he local culture of the community can be considered as icon. Any building which uses effective system of water management, energy saving, treating with wastes in an environmental friendly way can be considered as icon. Any building can provide users with visual and thermal and acoustic comfort is an icon from the functional perspective.
This is a very tough question. In my point of view, there is neither iconic architecture nor building. It depends on the recipient, culture, society, technology and so on and so forth.
If you think of an Icon as a product of a culture, with function and meaning, you may have one perspective or rather, if you think of it as a producer of culture, a start point, you may have a distinct perspective.
In the early modern architecture there were iconic buildings, because of the small number of buildings, and the existence of a period of time between the construction of a building and another, so can not compare the beginning of the century with the end, where the distinctive buildings sprout like mushrooms in the speed of construction
thanks, Dr.Amaar for sharing this interesting question
if you ask me this question before my study of this architectural trend may be my answer will be Opera Sydney House because it drew as an icon in my mind since the first time I saw it
Today, my answer should be more accurate
I can explain this complicated matter by putting the question about how can we judge this is iconic or not?...
always when we talking about iconic building our talk depends on the image of this building even though we study it in critical and analytical studies ...so our judgments related to the product image. Therefore, here we can put a long list of iconic buildings for the 20th century, but what are the limits of this list?!
in my opinion, there are no limits because literally, it depends on the taste of the spectators which never completely meet (as well as many other factors)