Catching Fire builds upon the world of Panem.
Catching Fire is the best book in The Hunger Games series because it builds upon the first book's universe. Although the first book introduces a lot of characters, most are in the series for a short amount of time without much background because of the Hunger Games. In Catching Fire, the setting of the Quarter Quell introduces characters with more elaborate backstories since they are all previous victors.  This expansion of Panem makes the book far more enjoyable than The Hunger Games or Mockingjay. In addition, the stakes are much higher in this book. Readers are captivated by a seemingly familiar plot with the annual Hunger Games as the setting but they are surprised by the heightened suspense. Katniss is at risk of losing so much more than only her life in this book. As a result, her character growth is greater than in the first and third books, turning her into a leader fit to face an uprising.
The stakes are not much higher in this book when compared to The Hunger Games. To some readers, it may feel like exactly the same plot. The setting of the Hunger Games again makes the book seem dull and unimaginative. While there are new characters, it feels like they are simply replacements of the tributes from the last book. Although Katniss' character has grown some, she is still a pawn of the Hunger Games. Even though she has the mentality of a leader, everyone else treats her as a passive component of the uprising. Despite Katniss growing as a character, she is still obviously held back back the other rebel leaders.
[P1] Catching Fire creates a more elaborate universe and introduces new characters. [P2] Catching Fire raises the stakes and suspense for the reader. [P3] This engages the reader much more than the first or third book. [P4] Catching Fire is the best book in The Hunger Games series.