A male Umbreon + a female Espeon, while cliche (and popular in this thread, too), I'd still have to choose it. Just a perfect pair.
I was also thinking a male Vaporeon + a female Flareon. Would be kinda cool, considering the colour contrasts, different genders, size differences... Just general opposites of each other. I think of it as Vaporeon protecting Flareon. :]