First of all, I found a similar topic but it's 1-year ago and a warning suggests to me to start a new topic. I apologize in advance if I made a mistake.
When I read the series I always tried to guess where the story takes place. I made some remarks that make me think that the city most likley is San Francisco. They live in US, on the West Coast, 'cause they have mountains near the coast. There are forests and a fauna typical of those area, and there's a river that runs through the city, and it's described as a very big city, with 60-stories buildings, theme park, water park, subway, a Marriot hotel close to the beach. And in book 38 the local newspaper is called "Chronicle" and there is a San Francisco Chronicle, and I believe that it is the only large city on the west coast to have a newspaper by that name.
Obviously, specific places (like Golden Gate or the Bay) are never mentioned and other places are invented (Beane Tower, Zoo Garden Theme Park etc.).
Few days ago I read a summary of book #54 and I discovered that California is indicated as the State where animorphs live, so my hypothesis is partially confirmed. Between the major cities of California, San Francisco is the one that most fits with the evidences in the books.
On the other hand, some other clues suggests otherwise. In #51 The Absolute Marco says that their state capitol is 350km away...but San Francisco is much closer to Sacramento. The cities at that distance on the coast are Eureka and Santa Barbara but they are too small.
Maybe K.A. Aplegate wanted to shown a generic big city in central or southern California but she didn't intend to refer to a city that actually exists.
What do you think?