Whose skills to max next?

I would say :
Archer : Robin / David (it depends of your team)
Lancer : Cu proto
Rider : Medusa/ Alexander (it depends of your team)

1: David 6/6/6
2 : Ushiwakamaru 6/6/6

David is one of the best all around 3 star caracter. First skill to level up is Harp of Healing.

Ushiwakamaru is a excellent boss killer, more versatile and team oriented than Robin (in my opinion). Meteor Horseshoes are painful to get, but the skills are worth it.

Euryale may be a good pick too, but there are just too many good archers in this game... Even if her niche is great (male killer), I realise I'm almost never using her.

N.B. Materials are just too scarce in this game, don't level up skills after the level 6 threshold. Unless you really plan to raise to lv 10.