Sekiro for sure
I’m playing Sekiro right now and loving it so I’d say go with that one. More people need to play it!
i love both, but would recommend playing Sekiro first. Nightreign has a playable character whith mechanics almost similar to Sekiro, and i guess it would be easier to adopt them after (not necessarily tho). also Nightreign is designed as an online game. even tho it has amazing story and a it’s even possible to play solo, it has a different vibe comparing to classic soulslikes