Kids popular books can be hard to narrow down on as each one has their own preferences. Therefore, to each its own.
However, I'll talk here from two perspectives -
My niece - My niece is loves everything science and fun. As strange as it may sound, she is a big nerd for a 9-year-old. She loves to read about the latest discoveries and trends. That's why her favourite book is actually a magazine that is National Geographic Kids Magazine. She is also inclined to mythology and history. On her last birthday, I gave her the Ultimate Collection from Amar Chitra Katha (https://www.amarchitrakatha.com/in) and she seemed to be the happiest to receive it.
My childhood - I loved reading comics as a kid. Therefore, I loved Tinkle the most. From Suppandi to Shikari Shambu and Angad to Kalia the crow. I loved them all. I had the oldest issues of Tinkle until the Mumbai floods. Tinkle is the best book to help kids get habituated to reading. Because Tinkle is where learning meets fun.