Well gonna go give my opinion for dark, you only really have 2 (or maybe 3) options to choose from, since the future good ones like Cherno or Berith aren't out yet
Satan - Probably the most obvious choice, pairing her up with Thanatos would mean you would be able to full burst faster cuz of her high combo atk rate buff, plus she gets an awakening next month which makes her even better (though you do have to wait for a few days for that)
she also has that handy orb eating move when you need an extra turn, and once awakened it turns into something like what cthulu has (i forgot how much burst gain she gets from it)
Osiris - 2nd choice probably, not the strongest option but being able to heal and provide +15 BG to everyone is actually quite nice to have, plus she doesn't die easily because of her invulnerability buff. And who knows? Maybe she'll get an awakening soon that will turn her even more useful in DMM (im one of those that hopes this actually happens)
Susanoo - I don't even know if she should be here, besides doing a large amount of damage with her 8x ability when awakened.. she doesn't really provide that much, and I bet people would even tell you to replace her with Chernobog or Berith when they arrive...
and if you also plan to get Herc's axe, then reaching def debuff cap isnt even a problem anymore, you might even think of replacing Thanatos/Osiris with Cherno or Berith
Also can't really recommend Hades lol, even though she seems more useful to me when awakened, a lot of people will shoot this idea down