You want a Water Gun/Hydro Pump Vaporeon for sure - it's a solid general attacker and eats ground/rock/fire types. It's good against Tyranitar raids, and Rhydon/TTar gym defenders, and Moltres soon. This might be the most important eeveelution - save the Rainer name trick for at least a 90% IV.

You want a Jolteon. Any is fine, Thunder Shock/Thunderbolt is marginally best. It takes out Vaporeon and Gyarados gym defenders, and flying - meaning the legendaries except Zapdos. Likewise save the name trick for a really top-quality Eevee.

A Flareon with Fire Spin/Overheat is the best fire attacker right now, and kills bug/grass and Articuno OK, but neither of those is really that important. So it's less useful in the meta right now, and will be less so after Moltres comes out next week. They're good to have but not a huge priority - take whichever random ones you get when evolving.

Umbreon is a lacklustre attacker, but a good defender. Defenders aren't important right now - get one for the dex entry once you've got the others covered.