I got It a 100% Eevee in research tasks and i dont know what to do, wait for next generation or evolve to espeon because have the best stats?
Depends on your current squad and goals.
Very new or rural players with limited access to legendaries should go for Flareon or Jolteon, which are the best available non-legendaries for their types (Charizard with BB is legacy).
If around level 30 with some decent attackers, but not yet able to solo Machamp, go for Espeon.
Have multiple high-IV, high-level Machamps, Tyranitars, Dragonites and legendaries? Then save the perfect Eevee for the next generation.
Before Mewtwo I would have said absolutely yes because that's what I did with my first 100% Eevee. Now I'm saving the other one I have for Sylveon. Yes, it's a long time away, but that's the next really special one. Glaceon will be great but outclassed by Mamoswine by a huge margin and Leafeon will be great but neck and neck with several other great grass attackers. Sylveon will be one of the best fairies, so that's what I'm saving my best Eevees for.
In short, if you have a Mewtwo, you will never use Espeon again, so wait for gen 4 or, if you're patient enough, wait for gen 6.
I have 5 Mewtwo. I have a perfect lvl 40 Espeon (and another one too). I use the Espeon way more - my best Mewtwo is slot 4 on my Machamp team and the others are 1 + 2. Also, slot an Espeon in between normal types for gym defense. My other perfect Eevee is being saved for Glaceon with a 96 for the poorly named Leafeon.
Slight disagreement - my Machamp team consists of Espeon (L36 Cf/Fs), Mewtwo (L30 C/P), Mewtwo (L36 PC/SB), Ho-Oh (L31, EX/BB), Rayquaza (L25, AS/AA), Lugia (L25, EX/SA). No reason to power up the flying Rayquaza or Lugia, they are just there in case I face C/DP when it's cloudy.
Now, if I get a 3rd Mewtwo, the Espeon might get dropped from the team, we'll see, but I think I'm more likely to have it with PC/SB as a raid generalist. But Espeon can still get you ahead of the clock nicely, especially in the leadoff spot where it takes a little longer for the boss to get the charged attack off.
If one had multiple Mewtwo that might be the case. Odds are if someone powers up multiple Mewtwo they will serve different roles unless that have an extreme surplus of them. I can't see someone justifying multiple Psychic Mewtwos over multiple Shadow Ball or Focus Blast ones. Espeon still has the second highest non-legend attack stat for psychic types and none of the legends above it have access to Future Sight.
The rest of what you said is pretty accurate though. Even if Glaceon is good Mamoswine will likely be better than it. If Leafeon is good it faces competition from a plethora of grass types, namely starters with Frenzy Plant.
Just do whatever you want. What's more important to you? I turned my 100 into an Espeon. If you get a 98% or a 96% for gen 4 it's not going to be a bad mon no matter what. 100% mons are just vanity trophies for the most part. Hardly different from a 98 or a 96 beyond telling everyone you got a hundo. ...Which is cool, but practically speaking it's not enough of a difference to make that much of a big deal out of evolving it.
I think the best choice is to evolve into Espeon. Second best is to hold it for if Glaceon/Leafeon end up being great and if not, keep saving for Sylveon.
Gen 1 eeveelutions are good attackers if no access to anything better, but they get outclassed massively by other nonlegendaries as more generations roll out. Flareon and Vaporeon in particular only fall down further as more starter community days happen.
Umbreon is defensively oriented and thus not worth consideration.
Glaceon has a small chance of becoming the best of its type. Seems unlikely that it would be given a good moveset but Mamoswine and Weavile were not.
Leafeon's attack stat is on the lower side, there's a monster called Tangrowth in gen 4 and Sceptile CD down the line is going to outclass every other grass mon by a huge margin.
Espeon has the second highest attack stat of its type out of all generations, it's a lasting investment into a good psychic mon.
If I were to get a 100% Eevee, I'd be waiting for Sylveon. It's about the only Eevolution that is unlikely to be outclassed by much. But then, I've got two Esperons at IV 96/98 (Both Attack/Stamina 15), my Dex Entry Umbreon, and... uh... oh wow.
So THAT'S what taking up space in my Pokémon Storage.
I've got way, way, way, WAY too many Jolteons, Flareons and Vaporeons. Time to get vicious in transferring! (or maybe seeing if there are new players who want a few Level 30+ish 1st Gen Eevolutions...)
Given how easy it is to go from Level 1 to Level 30 now, I'm pretty sure that most of the new trainers I know will be at a high enough level that the difference will be more about IV than Levels.
Heck, yesterday I realized that I had gained 7.3 million XP in 5 months. 7.3 MILLION!!! I thought I was only slightly higher than Casual, but 7.3 million since February 24? That is almost enough to go from Level 1 to Level 36. So I'm not particularly worried about trainers who don't have an army of Vaporeons (I've got 14 of them, all of them from 2016 and early 2017 - back when they were a great Gym Defender) or Jolteons (They were great for taking down Gyarados and Vaporeon) being too low of a level to get a benefit from the trade.
(seriously, 7.3 million XP?!? Makes me wish I actually had tried to grind out with Lucky Eggs more often; I'd be at level 40 now. Not that Level 40 matters anymore, but it's a nice milestone to reach.)
Fair, I forget how quickly people can level up now.
This week has been a massive boon for me level-wise. I was able to shotgun 22 Articuno raids under lucky egg effects as well as keep one going during all of Squirtle day and netted over 1 million XP in what essentially amounted to SIX HOURS of play. It was enough to push me over to level 38 and take me 1/3rd of the way to 39. Getting to 40 isn't high on my priority list but I did want to break 38 so I can max stuff out completely.
I started day 1 of UK release (July 14 2016). Year 1 I made 3.8M experience (So just into 33). Year 2 I've made 15.3M experience and it's not quite over yet (900K shy of 40). Now, I did move to a very very spawn rich place in the same town with better gym access...but it just goes to show you how much easier it is with events etc. I've made about 350K a day this event alone.
