It mostly depends on what team you're using and what skills you have available. If you're someone who changes team each event, it will vary as your options and needed skills change.
In general, Mordred is usually a top priority for her Black Propaganda, along with D'Art solely for Sniper Shot. Belphegor and Cybele have a weaker atk/def b-frame debuff which might be enough for regular content. So if you're running water or wind, you can hold on D'art because everything besides Sniper Shot that she has to offer is awful. Mordred does too but it's one of the lowest effect of the ones above but can still be used in the cases you're not running either of those elements or want to grab a different soul. Joan would probably be next in line.
Ryu Oh is very good, she basically removes the need to use Mordred.