Who should I promote?

I say Ares. +Atk-Hp is great, and he's a fantastic unit.

I would go for Arvis. Even though I have most of the non-seasonal red mages, I usually only use Arvis, because he brings so much to a team with just two extra skills inherited to him and can fit pretty much anywhere.

Hard to argue with Ares or Arvis. They are both great units.

Also, consider Lyon. Give him TA and Swordbreaker and he does a great job vs. green, colorless and sword units.

Depends what is needing in the roster. Aside from the suggestions already made, excellent units include: Kaze, Olivia, Caeda (mainly if +atk), Nanna, and M.Corrin if you have a specific unit you want to buff like crazy