The best Eevee evolutions in Pokémon GO as of 2020 is a slightly more complicated question than it seems. Because none of Eevee's evolutions are best in class for their type, the right one for you can vary based on the holes in your collection.
Best Eevee Evolution Pokémon GO 2020
The best Eevee evolution depends on what you hope to use the Pokémon for. The general consensus for raids is that Espeon and Glaceon are the best options. Espeon is the best Psychic-Type Pokémon outside of Mewtwo and Alakazams with a certain legacy move, while Glaceon is arguably better than every Ice-Type Pokémon not named Mamoswine.
For player-vs.-player considerations, the best options are Umbreon and Vaporeon. Umbreon is particularly useful in lower PVP leagues, with its utility dropping steadily as the competition gets more heated.
Finally, Leafeon and Vaporeon are strong candidates for taking on Team Rocket GO.
None of Eevee's evolutions are weak in their category, so players should use their Eevees to fill out holes in their collections rather than try to perfectly optimize.