Any legendary is good but what IVs/stats make the breakpoint on legendaries? It kind of irks me when the attacks aren't 15 but maybe I'm wrong?
15 attack is valuable for
a) minimizing the amount of stardust it takes to reach breakpoints
b) minimizing the chance that your mon misses a breakpoint even when maxed out
a) is very relevant if you don't max those pokemon, while b) is unlikely to ever matter. I'd say 13 or 14 attack is generally speaking adequate, which is why I'll power up a 13-15-15 over a 15-15-10, but this is a matter of personal preference.
As detailed by the above posters the main considerations are if you're maxing something out or just powering up to the low level 30s to hit breakpoints.
I would also like to add that the base attack stat of the pokemon and the move pools should take into consideration:
Generally speaking, the lower the base attack stat of the pokemon in question, the 'more help' it needs. Examples include Articuno and Lugia gain a lot from extra attack, since they are just under 200 to begin with.
After looking at base attack stat you then go into the move pools. For Kyogre/Groudon for example the attacks are the same at 270. Take note that Groudon's best fast move has 5 power (mud shot) and Kyogre's best fast move has 16 power (Waterfall).
Kyogre will hit more breakpoints but each breakpoint it hits is of minimal gain. Groudon will hit fewer breakpoints but each breakpoint will represent a huge damage gain (e.g. if you go from 5 to 6 damage, you gain 20%. If you go from 16 to 17 damage, it is under 7%)
Kyogre does hit a breakpoint at 15 attack vs Entei when maxed out at level 40, at which point Waterfall goes from 20 to 21 damage. Significant? Up to you to judge.
