Which eeveelution?

Looking at the remaining Gen 3 legendaries (not including Jirachi and Deoxys since they're mythical), we have the Lati twins and the Regi trio.

Against these, the only Eeveelutions that stand out are Flareon (for Regice and Registeel) or Vaporeon (for Regirock), though Vaporeon gets highly outclassed by Kyogre.

Personally, I'd wait it out, see what Niantic throws at us next, and then make a decision based on what you feel you'd need most.

P.S. have you already used the nickname trick for forcing your Eevee to evolve into one of the specific forms? If you have for the gen 1 Eeveelutions already, then I'm afraid you'll have to leave it to fate if you want a Vaporeon/Flareon/Jolteon, as the nickname trick only works once.

Since you have enough Psychic-types and Kyogre, but lack Fire-types and Electric-types, pick Flareon. It is a better generalist than Jolteon (was a great Blissey counter during the CP-creep era) which will be useful in Regi raids and for attacking Metagross (even Gardevoir) in gyms. Only consider Jolteon if Gyarados is a common gym defender in your area and/or you also lack Grass-types to take on Water-type defenders/raid bosses (note that Grass is neutral against Gyarados).

I just caught a 100iv Lv 35 today. Only 88,000 stardust to Max it out which I'll do. To an Espeon cause I want that best 300CP.

Id hold onto it for a perfect Sylveon or Glaceon later