I know people dont like him here, but I think Gin could be a good substitute for Aria when it comes to trying to do TOA. Obviously he's not as good as her in any way but his aoe glancing hits could help out a lot in mid-high levels of TOA since there arent many other mons who provide that. Other than that, I don't really think he's very useful.
Luna is a decent mon and I can see the potential in her for AO but the problem is that she's very situational. She really only comes in handy against Bella and Darion in arena and I feel like the light comps are starting to go away very slowly with the introduction of Veromos (and maybe even Dias). I dont see much usefulness to her anywhere else. Also, owners of her seem to be underwhelmed with her so that's a negative sign.
Delphoi is a good mon, but I am not really sold on her. Her immunity and cleanse is great but the problem is that there are other mons who can do the same things she does (Draco, Fedora, Chloe, Woosa, Velajuel) Maybe I'm biased because I have Veromos, but I feel that his cleanse every turn is very irreplaceable and viable. A downside to Delphoi is that you have to wait for her immunity to come back up again (not sure how long the cooldown time is) This is not really the best for TOA or Dragons because frequently it can only take 2-3 turns of the enemy to put dots on you for you to get killed and you have to remember that like Ahman, if she gets stunned at a important stage of the fight and can't buff with immunity, it could be game over. Yes, it may take you a long time but if you spend so much time playing this game already what's another 2 weeks (at max) of fusing Veromos really going to do? A great plus side to Delphoi is her first skill that resets cooldowns which is very valuable in arena if you can build her with enough speed.
Also, I've seen the argument against the idea that Delphoi is obtainable from MS with most people saying its like the chances of pulling a Zaiross, Perna or Zeratu. Really? She's a nat 4* not a nat5, you cant compare the two when the summon rate is clearly nat4 > nat5. Eventually, you will get all the nat 4 on the list of water, wind, fire attributes (yes, even that chloe that you've been trying to pull for the past 8 months) It may take 2 years to pull the one that you want but you will pull it eventually. Take a look at one of YDCB's videos on "how to rune". He literally has every nat 4* (except for the newer ones). Even if you're free to play, it may take you even longer, but you will get them eventually. I know a lot of people will stop me here when I say eventually by saying that eventually will not help with game progress but honestly, I think this is what keeps the game alive. Not getting that mon that will help you pass a certain level means that you're going to get creative with the mons you have in order to find a way around it. (and then you can post it on reddit to help the rest of us out who havent been as lucky as well :D)
So TL;DR: I have no idea who I'm going to pick xD