What to do with Eevee?

Yes.
It appears you have the question and the answer.

Congrats on a very nice 100%.

Yes, congrats!
Third perfect eevee!
You're a lucky, lucky guy.

Espeon.

If you don't 'need' any of the current evolutions, I would consider waiting for Gen 4; a guaranteed evolve with a 100% Eevee would likely be more useful.

Congrats on your catch. I am a bit N-eevee-ous.

If you haven't already used the Rainer trick, why not go for a third Vaporeon? They're still the best water type in the game currently.

But if you already have used it, maybe see if the Gen 4 Eeveelutions are any good and also if they're a guaranteed evolve.

You're right about the other Eeveelutions being outclassed.

I don't have a 100%, but I have quite a few eevees from level 20-30 with ivs from 80%-98%. I have all the eevolutions I need. Plus, jolteon and flareon have both been outclassed twice by legendaries. Espeon still has the advantage of being much more accessible than Mewtwo and more dust efficient than Alakazam. If you don't have a couple, go for it. Maybe someday Umbreon will be more useful. For now, I'm saving my best eevees for either gen 4, relevant umbreon, or moveset rebalance.

Articuno / Regice / Mamoswine >> Glaceon
Tangrowth / Shaymin >> Leafeon
Togekiss / Florges >> Sylveon

So you might as well just transfer it now.

/s

I'd be pretty tempted to save him in case Glaceon or Leafeon have great stats. Evolve him now and there's a chance he comes out as an Umbreon and that would be heartbreaking.

Go with espeon. Solid psych attacker and can be used against everthing with no resistance. Sure mewtwo is better but we dont know if there is a Chance for more then 1.

I'd personally go Espeon, since (mild bragging) it's the only current perfect eevolution I don't have. But it might actually be worth holding onto for Leafeon or Glaceon if the current stat calculations for Go remain the same, Leaf has the highest Def among all the kitties, and the second highest Atk (20 less than Flare), while Glace ties Espy for highest Sp Atk, and Umbry for second highest Def (20 less than Leaf).

Very nice. I hatched a 97%. I actually maxed it out all the way. I may hold out for glaceon but don't have to decide right away. Also the Eevee theme gyms are in for a surprise. They are very popular in my area. I absolutely have the strongest Eevee around ?