My favourite media of all time is the Digimon franchise
I got into Digimon when I was 6 years old via DVDs of the anime seasons and they really had an influence on the person I am today. What set them apart from other medias I was engaging with as a kid? I'd have to say the story-telling and the themes it carried throughout its seasons in comparison to other children's media I was engaging with at the time. In particular I think they showcased grief and people's reactions and ways of dealing with it well. It is important to note that grief when it comes to Digimon is different because of the idea of reincarnation being applied to them as Digimon are just files of 'data' so when they 'die' they 'reset' and become a digi-egg to hatch again into a digimon but the newly hatched digimon will have lost all of its memories (a nod to memory data) -- this doesn't take away from the grief of the initial digimon death because that digimon will never be the same exact one and there's no guarantee of the digimon and human meeting again, in fact it's kinda worse when they do because the human has retained memories and will feel a different kind of frustration and grief and it's interesting how that is explored.
I think what kept pulling me in throughout the years was how the season 1 main cast was brought back in the seasons that came after and with every season they'd age and I was watching this cast grow alongside me, I remember being a kid and they were canonically kids and now I'm an adult and they're also canonically adults it just makes it a bit closer to my heart.
However, I don't think my favourite media of all time is the best of all time because I'm critical of its writing and messages here and there and the quality of their older games are so bad.
The media piece that I think is the best of all time is Witch Hat Atelier and
To end my answer I want to tie it off by saying that I think my two favourite media pieces tie in together as a progression of childhood to adulthood