Rayquaza power up question

The one with 15 attack and defence.

When defence IVs do come into effect i.e. due to enemy quick move breakpoints, they make a bigger difference in battle than the difference in HP IV which is usually only 2 or 3 HP which might not do anything at all as raid bosses usually overkill you.

My best Rayquaza is 15a/12d/15s, but if I had 15a/15d/10s, I would choose that.

Totally agree with above posters. The extra defense will save you more HP than the HP lost from lower stamina. No brainer, go with defense. Now, if it were 15/15/10 vs. 15/13/15 (a/d/s), that might take some calculation. But with your choice, 15/15/12 is the clear win.

I strongly disagree, let your Rays run through some calculations and see that the 15-12-15 is in fact the better option.
Breakpoints are heavily overrated, if you plan to max out your mons anyway, what it looks like. Those extra 2 HP (cause 14 IV=15 Iv on hp at L40 with Ray) can make a difference in gym battles, as it might safe you a revive, which is nice if youre short on them anyway (like me lol).
With 12 Def IV you will cover most breakpoints anyway.