Which Eevee evolution?

Can't go wrong with any of them, but Espeon is widely regarded as the most succesful and relevant attacker.

Vaporeon would be a big help at least until Adventure Week is over. He's super effective against most of the raid bosses out right now

What lvl is the Eevee at?

If its lower than lvl 20, get Espeon. Its the only worth powering up right now.

If you have it at a good level then you could probably get your favourite, they are all good except Umbreon. Especially since you recently started you probably dont have any legendary

Psychic is important as you work toward Machamp raids. Espeon is one of the best psychic attackers. I still use 3 of mine. I never use the others because I have better fighters for all elements.

Dont evolve it now.

Wait until generation 4, both those evolutions are very strong, and most of the other evolutuons are kinda outclassed by other pokemon

Glaceon's really the one to save for, its an ice type that has actually decent defenses AND huge attack (on par with scizor, belloom and some of the other evvee's) and is an ice ty[e that ISNT paper thin

But as for right now, Jolteon would proabably be the best, its the best electric attacker that isnt Raikou, (though that could be taken by alolan golem/raichu when those events happen) so realistically there is NEVER a 'best' option for any of them when evolving, they all have their uses and purposes, just depends on what your playstyle is

If you just started playing your eevee shall be low level. So I'd say evolve it randomly and use it in gyms. When you get high level you must find better level ones that would save dust and candy.

Unless the stats are really amazing (over 90%) don't make it an Espeon as that takes work and investment.

Also absolutely do NOT use the cheat names until much much later when you find amazing examples.

I say you should just do a random evolution. All three options are great to have as a new player and are the best of their types, until you get legendaries.

Except Umbreon, all Eevelution is good for their typing, not the best but good to have if you dont have best pokemons at their typing.
If cost of stardust and candy (and probably TM) is not a problem and you like doing a battle, i will choose Espeon for its usefullness in curent meta
Get a Confusion and FutureSight moveset, and it is a great addition to your squad for soloing Machamp, Espeon is one of the best in non legendaries psychic competing with Alakazam.
And Machamp has many use in soloing tier 3 raid boss right now, very good counter against the future Regi trio,and the best in duo a Tyranitar, another meta relevant pokemon if you cant play with many other trainers

Get Umbreon (name trick applied: Tamao) for the dex entry, then don't evolve to it ever again.

There really is only one sensible option: Wait until Toxeon is available! Who wants to be hasty and settle for something that is not the best?

Need more information.

1) What do you mean by "Pretty Good IV Eevee"? To me, "Pretty Good IV" is above 82%, but I'd need to know what you mean by "pretty good IV" before I answer.
2) How many Eevee's can you find in your area on a regular basis? When no Event is going on, in my Area I typically find about 15 Eevee's a week (more during Party-Cloudy weather) without actually grinding. Because of this, I can afford to wait until I get a 90%+ and/or Level 30+ Eevee before I evolve it.
3) What are your needs right now on Attack and on Gym Defense? Jolteon and Esperon are the top two Gym Attackers for Eevee's right now, both of them are only outclassed by Legendary's (for the most part, anyway). Flareon is pretty good for both, as is Vaporeon, and Umbreon is a Dex Entry. (insert sad emoji here) So if you need an Electric or Psychic Gym Attacker, Esperon and Jolteon are the ways to go. If you need a Defender, I'd go with Vaporeon, but might also think about Flareon - in my area, I can toss either of these on any gym I want because no one uses them anymore for ... well... anything, it seems. So they will be useful on Gym Defense at least as something you can reliably put on there (unlike, say, Snorlax, Blissey and Chansey) and get a decent bang for your Pokecoin.

Now I'm really thinking there will be an Eevee community day as part of the Let's Go, eevee hype train. Collect more and more of them when you see a few clouds in the forecast