Well it's extremely hard to say because one is just a weakness to other,Eifie/Espeon is weak to Dark Blacky Umbreon,Leafy is weak to Glacia,Vaporeon is weak to Jolteon (which actually doesn't have a weakness of the Eievui evolutions),Flareon is weak to Vaporeon,aye..I it depends which way they were battling,what attacks they have,what levels they are..so I'll give Eievui the benefit of the doubt here.

You are right but then again, Eevee is only the firs evolution that makes eevee weaker, so maby Jolteon would win.... :/ I just picked Vaporeon cause I wanted to... :D Oh and not only that, since Vaporeon can learn ice type moves that would be a advantage against Leafeon, and I don't about you guys but my vaporeon doesn't take a lot of damage from electric type moves..... so.. yep :D

With its speed. Or Vaporion who can Hide under water!

Umbreon has the advantage like Jolteon of having no weakness type-wise against the others as well as having a type advantage against another.

Oh yeah I forgot Vaporeon blends in the water! Ooppss

And wwooww so many people picked Umbreon .........

Jolteon+Thunderbolt+Vaporeon hiding in water=fainted Vaporeon

Well, maybe the Vapoeron would be smart enough to realize that going in the water would be retarted because Jolteon is obviously an electric type.... I'm pretty sure pokemon are smart! :D
And like I said my vaporeon can stand up against electric type moves........
And nice using a math problem to say that, a guy in my class did it and it was sssoo funny the way he said it.... now that was random. :D