Which architectural designs become 'city landmarks'? Why?

This is an interesting question. In my opinion, for a building to become a "city's calling card," merely being beautiful or quirky isn't enough. It needs to possess several key characteristics:

First, it must have extremely strong recognition, making it instantly memorable. It's like a person's appearance; some you meet and forget, while others leave a lasting impression. A building that becomes a city's calling card definitely needs a "masterpiece" in its design. Take the Sydney Opera House, for example – does it look more like shells or sailboats? The fact that people can discuss it makes it a success. Or Beijing's "Bird's Nest" stadium; just hearing the name conjures an image in your mind. This unique design allows it to stand out among thousands of buildings, becoming a backdrop for countless photos and check-ins.

Second, it must have a "story" and "substance," capable of representing the city's spirit or history. A building isn't just steel and concrete; it's a vessel that holds a city's culture and memories. For instance, the Eiffel Tower in Paris was built for the World's Fair, symbolizing the industrial revolution and technological confidence of its era. The Forbidden City in Beijing represents centuries of imperial history and Chinese culture. Such buildings are deeply intertwined with the city's history and culture; seeing them immediately brings to mind the city's "character." They cease to be mere structures and become cultural symbols.

Third, it's best if it's "alive," public, and accessible for people to experience. This point is crucial. If a building is just a private company's office, and you can only admire it from the outside, it remains distant from the citizens. But if it's an art museum, an opera house, a library, or even a bustling transportation hub, it's a different story. People can visit exhibitions, attend concerts, go on dates, wait for others... The building becomes part of countless daily lives, bearing their joys and sorrows. In this way, the building gains warmth, becoming a shared memory for everyone, and naturally, a "local" calling card.

Fourth, it should harmoniously coexist with the urban environment, and even revitalize an entire area. A brilliant design doesn't stand in isolation; it considers its relationship with surrounding streets, plazas, and the skyline. Sometimes, a landmark building can directly boost an entire district, much like the Guggenheim Museum in Bilbao, Spain, where one building revitalized an old industrial city. It's like throwing a stone into a calm lake, creating ripples that bring the entire environment to life.

So, to summarize, a building becomes a "city's calling card" typically because of its distinctive appearance (recognizable), rich substance (cultural depth), ability to accommodate people (public space), and good neighborly relations (integrated with the city). It's like a city's superstar, possessing both beauty and strength, along with approachability.