Which Harry Potter game do you think nailed open-world exploration the best? "Harry Potter: Hogwarts Mystery" has been my go-to for exploring every nook and cranny of Hogwarts.
"Hogwarts Mystery" does a great job at letting you explore the school and meet various characters. However, I also had a blast with "Harry Potter and the Half-Blood Prince" for its open-world exploration of Hogwarts and Hogsmeade.
