Shingeki no Kyojin - When Titan aired in 2013 it was big but even then I don’t think anyone really knew it’s potential. There are so many things that were setup and only paid off now nearly a decade later and it’s all been worth it.
Sword Art Online - I was pretty somewhat into this until the end of the first arc. I don’t think it holds up but it does represent a very interesting time when VR still hadn’t caught on and the upwards trend of Isekai was still young.