Which Entei to power up? I live in an area where the weather is clear 80% of the time

My vote is for the 1901. Though the 1896 is 15 attack and the others are better IV, Entei has high HP, and will benefit more from Attack and Defense. Having good Defense on it will give it more longevity. The highest two are pretty impressive, but they prioritize HP, which Entei doesn't need quite as much.

1896. U want 15 attack. Might not be able to hit last fire spin breakpoint at 40 on a future legendary boss with 14 attack. I'm not sure yet since the legendaries are not out yet and they might get a nerf.

I would go 1901 as well. Best combination of Attack and Defense. You won't be hurting for health.

Only 15 attack ones since thats all you need.