1. Free! - Iwatobi Swim Club (2013): The first season of the anime, introducing the main characters and their journey in swimming.
2. Free! Eternal Summer (2014): The second season, which continues the story from the first season and delves deeper into the characters' pasts and motivations.
3. Free! - Take Your Marks (2017): A movie that takes place between the second and third seasons, focusing on the characters' experiences leading up to the Inter-High competition.
4. Free! - Dive to the Future (2018): The third season, set a year after the second season. This season focuses on the characters as they graduate high school and navigate new challenges in their lives.
5. High☆Speed! - Free! Starting Days (2015): This is a prequel film that tells the story of the main characters' journey into swimming during their middle school years. It's best to watch it after you've finished the first season.
6. Free! Road to the World - Episode 0 (2019): This is a short OVA that serves as a bridge between the third season and the upcoming fourth season.
7. Free! The Final Stroke (2021-2022): This two-part movie series is the fourth season, focusing on the characters' aspirations and journeys in their post-graduation years.
Note: There is also a special episode called "Free! - Iwatobi Swim Club - Episode 0", but it doesn't offer anything essential to the plot. You can watch it whenever you like, as it's more of a bonus episode.
This order allows you to follow the characters' development throughout the entire series and provides a clear progression of events.