Which Rayquaza to power up, 15 15 13 or 14 15 14?

What ray is good for is DPS, so I'll go for the 15a.

TBH, I don't think any material difference between the two when maxed out.

If I’m reading this right both have 15 attack. I’d go with the one with the higher defense. You might lose one HP (and that’s probably it) when maxed out and there will occasionally be a battle when, due to breakpoints, you’d lose one extra hp per round. I only have one maxed out Rayquaza so can’t compare, but with two maxed out machamps - one with 9 HP stat and one with 15 HP stat - the 9 HP stay wound up with a total of 149 HP points and the one with the 15 HP stat wound up with a total of 154 HP points.

14 hp iv Rayquaza has same HP as 15 hp iv Rayquaza
At level 40
So 14 15 14 > 15 15 13 at level 40

(HP att def)