The most important thing is don't forget to watch the post credit scene in episode 25 of Season 1. necessarily need to watch the OVA "no regrets"- in 2 parts, it's canonical, and it's best to watch it after season 1, before the second season. This is necessary in order to understand the different references that were and will be further in the anime. And OVA "Ilse's Notebook" also. This is a mandatory program for viewing AOT! This is a must-watch program and for the best experience from the show. Many reactors wondered why these OVAs are not included in the main show.