Kill Bwonsamdi, because that guy is the worst (his dialogue is fun, but I don't think there's any character I've wanted to kill more in BfA, Sylvanas included).
Eliminating it down to just three, I think I'd go with...
Kiss Zul, because killing him is just going to make him come back as some angry undead, and I don't need that stalking me.
Marry Vol'jin, because I'm already exalted with Vol'jin's Spear.
Kill Rastakhan, because he can't be trusted to lead the troll people anymore.