Full Metal Alchemist Brotherhood had already been proposed and it was my first pick as it is the anime that for me started it all for real. SAO is the second long one I watched and it had also been proposed already, then Cowboy Bepop which is a masterpiece is also already taken.
What am I left with... it's a list, and not a short one... I'd propose Ghost in the Shell, a forever classic but it's a film and not a series. Thus... I have to pick something else... and I go for...
Sousou no Frieren (Frieren: Beyond Journey's End)
"During their decade-long quest to defeat the Demon King, the members of the hero's party—Himmel himself, the priest Heiter, the dwarf warrior Eisen, and the elven mage Frieren—forge bonds through adventures and battles, creating unforgettable precious memories for most of them.
However, the time that Frieren spends with her comrades is equivalent to merely a fraction of her life, which has lasted over a thousand years. When the party disbands after their victory, Frieren casually returns to her "usual" routine of collecting spells across the continent. Due to her different sense of time, she seemingly holds no strong feelings toward the experiences she went through.
As the years pass, Frieren gradually realizes how her days in the hero's party truly impacted her. Witnessing the deaths of two of her former companions, Frieren begins to regret having taken their presence for granted; she vows to better understand humans and create real personal connections. Although the story of that once memorable journey has long ended, a new tale is about to begin."
Synopsis from MyAnimeList
This is a masterpiece. It's long with 28 episodes, compared to the standard of 12, but it doesn't feel neither long or boring; they took their time to develop the story of Frieren and its companions.
https://www.youtube.com/watch?v=Iwr1aLEDpe4
It aired between September 29th, 2023 and March 22th, 2024.
Other animes that were running for the pick were Violet Evergarden and Haikyuu. And a few others as well but oh well.
