which eevee evolution is the strongest?

Competitive wise, Jolteon and Espeon hold high potential (all Eeveelutions can be good) however modest Jolteon works really well, even with its original ability, and I personally find it easier to use, so I'll say Jolteon.

Wish we had a dragon type eevee in sun and moon..

I think Jolteon is the strongest, but Flareon is my favorite.

Umbreon