I can't exactly rank them from the Alvarez Arc because they haven't even made an appearance yet. However, from previous feats...
- Cobra > Midnight [High Difficulty]
Cobra and Midnight are pretty much the strongest from Oración Seis if you don't count Zero. I'd say they are relatively similar in power but Cobra will take this in the end. Note, that I will not bring any sort of hype for Cobra just because he is a dragon slayer. That is not as important to me as others. Midnight, on the other hand, seems to have a lot of defects in his magic as well.
First of all, with Cobra's extreme sensitivity to sound, he would be able to counter Midnight's illusions as he can keep his eyes closed during the entire fight. We saw Erza negate Midnight's illusions because of her fake eye which caused her to be able to realize that the illusions she were seeing from her other eye were not real. Using this logic, Cobra can close his eyes and solely rely on sound to fight Midnight.
Second of all, we know Midnight can only refract or deflect one area at a time. Had Cubelios been included, Cobra would have taken this fight a lot easily. But this is just Cobra vs. Midnight alone. Anyways, I would think that even if Cobra's body was restricted, he can still use Poison Dragon Roar to hit Midnight. During Erza's fight, we saw Erza being helpless against Midnight at first because he was bending her armors. This makes sense because Erza relies on her hands + feet to fight. But like I mentioned before, dragon slayers like Cobra can use their head to attack without the use of their hands or feet.
Lastly, we know that Midnight can manipulate armors + weapons but not physical attacks. We have seen that Cobra can attack physically with dragon slaying magic so this also goes against Midnight.
Not to mention, Cobra probably outclasses Midnight in physical strength, durability, speed, endurance, and stamina. On the contrary, I can only see Midnight having the upper hand on magical power if that. They should both be relatively the same in intelligence.
**NOTE: You guys can argue that Cobra is not able to pick out Midnight's weaknesses like Erza can which I can see. In my opinion, he is not as reckless as Natsu and Gajeel but he definitely isn't as smart as Gray or Erza either. Therefore, I would place him fairly average in which he should be able to eventually find out about Midnight's weak points if the fight were to last. Also, because nothing states about intel here, I guess it can go either way or we can negate that.