Things Fall Apart by Chinua Achebe. It confronts European colonialism and Christian missionaries, but is also critical of aspects of African culture (toxic masculinity, certain violent traditions) in the 19th century.

It's very interesting and a book I revisit often.

The Percy Jackson series was my life in middle school! It's been a couple years, but I still think (from an adult POV) the characters are developed well, the plot is compelling and exciting, and it's a great introduction to Greek mythology.

As an early teenager, I appreciated the well-rounded cast of female characters. They're realistically portrayed and have dimension, which is hard sometimes in YA novels.

I definitely recommend it (even to adults who want something fun), just don't watch the movies.