Apo, Steelskull and Skullcrusher would all be good at 6*. Keep in mind that Apo isn't primarily a healer - his first job is speeding up your team. He does have a strong heal on a short cooldown, but it's only single target, so he won't heal as much as Steelskull. Good aura as a bonus. 

So the question is just in which order do you level them - not sure on that, but don't think you can go far wrong with any order you pick. 

For dungeons, the most important is dragon, because it offers many of the most important gear sets. Try to focus on progress there - with your team depending on affinity, but generally speaking I'd think SK, Septimus, Kael and two of the three discussed above. Poison is very strong on the dragon, which gives a little extra to Steelskull, but the others also have their advantages.