Which Moltres and how?

If you're like me and tend to ignore cost for 'mons that are particularly good or 'mons that you really like, I would raise 1 or 3, 4 if you value shiny Moltres particularly. 1 because it's the highest overall IV value despite the 14 attack (and being the best in the event of it becoming good in PVP), and 3 because of it being the highest 15 atk. I personally would rather have a sky-attack which can have overheat added as a second move.

On a more neutral view, 2 is probably the best. The difference in IVs is going to be highly negligible short of a breakpoint miss (which is fairly unlikely at lv40), and the fact that it's lucky will allow you to take 50% of your investment and save it for another 'mon.

Can't put shiny legends in gyms to show off, so the bragging rights are greatly diminished. I personally wouldn't bother powering up a shiny legend unless it was also one of my top IV candidates.

I would go with the 14/15/12 lucky. The 1 short on attack is not likely to matter, and the 3 less stamina is probably a 2hp difference. The cut to stardust cost would be dramatic though.

Moltres is not bulky by legendary standards, so 15 attack is not as crucial. Power up #1 for maximum performance. If you are short on stardust, #2 will do, but for legendaries, candy is the main constraint.

I don't see much use for several moltres, so I say go all out on the 97% one.
Buy a second move to get sky attack and overheat and level it up to 40.