100% Eevee

If you don't already have a good Espeon, then yeah do that! Otherwise, consider saving it for gen 4?

Espeon is a good idea. I have an 100% Espeon with Confusion, Future Sight. It's great, but also a glass cannon. Be careful - must evolve while as a buddy, right after 10K, and during in-game day time. My family evolved after changing buddy; got Jolteon.

OR

Save it for Gen 3, Leafeon or Glaceon (ice will be useful).

If you are looking for a defender or a generalist attacker, definitely Vaporeon.

If you are looking for an attacker and don't bother walking with Eevee, maybe Espeon.

If your gyms are most filled by those Gyaradoses, definitely Jolteon.

If you are looking for an attacker and quite bother walking with Eevee, definitely Flareon.

If your gyms are most filled by those Espeons and don't bother walking with Eevee, maybe Umbreon.

If you live in a water biome and almost everyone places Vaporeons in the gyms, maybe Leafeon, wait for Gen 4.

If you live in a water biome and almost everyone places Dragonites in the gyms, definitely Glaceon, wait for Gen 4.

If you live in a rock biome and almost everyone places Tyranitar in the gyms, maybe Sylveon, wait for Gen 6.

Yes go for an espeon and pray you get confusion.

If you already have good enough Eeveelutions that you are satisfied with, then wait. Note that Leafeon and Glaceon are not till Gen4 though.

Hold on to the Eevee and wait for trading. You should probably be able to get a nice return for it. I am holding on to my name trick for just the right Eevee and I am sure I am not alone.

This bad boy is powered up to level 35.5 and has been holding a gym spot for over 6 weeks. I have forgotten where I left him!