Lots of contenders for that top spot…
One I like a lot is Haibane Renmei. It’s not the usual fantasy-adventure story, as there are no knight and warriors, no quest to find a lost treasure, or other kinds of common fantasy story elements. But it is set in a different kind of world with some characters, the haibane, not being normal humans.
Why do I like it so much? One reason is because it has a slice-of-life feel to it, of people going through everyday adventures. But it’s not just that, there are deeper themes explored, things like fallenness and sin and forgiveness.
It’s an older series, I think from the early 2000’s.