Check out this list of the subways with the most riders. This is the top 10: Tokyo, Seoul, Beijing/Moscow (tied), Shanghai, Guangzhou, New York City, Mexico City, Paris, and Hong Kong. Here is how the story describes these subways:
While vital to both big-city residents and visitors, subway systems can inspire a love-hate relationship, with overcrowding blamed for much of the frustration. While we may not love riding in sardine-like train cars, we do appreciate the efficiency and even beauty of many of the world’s most popular subway stations.
I’m not sure why there is consternation about the crowded nature of these subways: are there more efficient ways to move millions of people in some of the densest areas humans have every known? If everyone could have their personal space, like in cars which Americans prefer, it becomes really hard to have cities with densities like those in the top 10. If we operate with the assumption that all humans would prefer to be in less crowded spaces if they could afford to, then this might make sense.
I wonder if such complaints in the United States about crowded mass transit betrays American sensibilities for privacy and space. While people in other countries might choose mass transit over the costs of cars (and they are expensive to operate, in addition to the space, infrastructure, and resources they require), Americans work in the opposite direction: they would prefer a car until it becomes too difficult. For example, see this discussion about getting wealthier Americans to ride buses.