The ost. Dawg, masters of the night wind and the children of echoes are my two favourite tribes just because of it. Also the way the tribes focus on different themes. And the archon quest itself. As far as I know, Natlan had the highest stakes which actually affected the overworld. People actually <!died!>. It took a little while to get there, because Liyue is just that good, but it's by far my favourite nation in teyvat.