Just like every team with any Ronia's involved the ideal sub is another Ronia.
I'm actually not positive long-term. The new Hera-Sol dungeon has a new mechanic designed to slow down Row/Heroes teams from steamrolling everything. "Combo Lock" where if you do less than 6 combos it heals the monster. For everything before that you'll just go rows I imagine. D/D Luci, Ronia, Tiamat, Loki, Gryps etc for dark rows.
Ronia(awakens, skill) and DD Luci(awakens, skill) probably tops of the list. Between them and Lu Bu/Ronia leaders you're at 5 rows and 7 skill boosts. If you're playing as a dark row team that's a pretty good start. I'd be tempted to bring Hanzo even though he's not a Devil for a full dark board(after Ronia) and 6th row enhance 8th skill boost. Slap a D/L Lilith in there for some RCV, 7th row, and move time. Ronias, Luci, and Hanzo can be skilled up, Lu Bu cooldown will be 8 turns after all the skill boosts. If you can't wait for that or don't need the full 2.5x then Lilith's(skilled) 1.5x will be up in just 2 turns.
If not Hanzo maybe D/L Batman when we get his Uvo, it adds Devil and he has a 2 turn delay and a dark row enhance.
A red row devil team doesn't have quite the flexibility. Shiva for orb enhance(long CD) and 2 rows, Belial 2 rows(and long CD green>red). after that the pickings get slim for really desirable subs.