Kind of depends on the game, but most of the time the ingame music. As it is the music that was intended for the game by the developers I like to at least try to use this for the experience. But of course sometimes the music just sucks :D
This is tough to decide. I started with DS1 and this was such an eye opening experience on this genre that I would usually pick that, although Bloodborne and Elden Ring might be better if I try to narrow it down more objectively for me. So I will go with DS1 -> Elden Ring -> Bloodborne.