2nd charge move on Rayquaza or Moltres?

I don't have either but can see myself adding a second move to Moltres before Rayquaza, especially for gym clearing.

Moltres with Sky Attack/Overheat can hit most defenders for neutral damage at least, even if single-bar moves are overkill for gym sweeping. Sky Attack is also very fast, improving your clear time.

Rayquaza doesn't get much of a benefit from a second move on gym offense. Outrage hits most things for neutral damage as is, and thanks to its sky-high attack stat doesn't need anything more than that. All three charge moves are resisted by steel types and I can't see someone willingly leave it out against fairy types.

I dont think a second move is necessary on either.

For ray, the only things that resist dragon are steel and fairy, and of that, only metagross seems to be a common sight in gyms (SA moltres is also a great flying attacker, and theres many better rock attackers than ancient power ray). Moltres, while a fire move lets it hit metagross for super effective damage, that is really about it right now, SA is better against grass and bug types, and theres steel raid bosses at the moment and for the odd few metagross in gyms, a good BB Charizard will do well, so you likely wont get much use out of them for a while (As someone who'se gotten a second move on my SA moltres, I almost never find myself using overheat).

I'd recommend saving the dust right now, and for rayquaza save the candy, we are somewhat due for a raid rerun focused on it with groudon and kyogre getting a rerun so recently

I've unlocked Ancient power for rayquaza 98%, but not moltres.

I only use it for gyms, against ice, fire and anything that is weak against rock.

Moltres obviously, but onky if it has sky attack on it. I have sky attack on my moltres and i actually find more use with overheat but sky attack is exclusive, so im keeping it for now. Rayquaza on the other hand, not very necessary because outrage is way stronger and all three moves are resisted by common opponents in pvp like metagross.