What's the best nature for each eeveelution on the game?
Depends on the role you want for each of them. Here's my usual go-tos:
Vaporeon - Bulky special attacker - Bold, Calm, Modest
Jolteon - Fast special attacker - Timid, Modest
Flareon - Bulky physical attacker - Jolly, Adamant, Careful
Espeon - Fast special attacker - Timid, Modest, Calm
Umbreon - Defensive wall - Bold/Calm for special sets, Impish/Careful for physical sets
Leafeon - Fast physical attacker - Jolly, Adamant, Impish
Glaceon - Bulky(?) special attacker - Modest, Calm, Bold
Sylveon - Bulky special attacker - Modest, Calm, Bold
Adamant and Modest if you want to hit hard, Jolly and Timid if you want to go outspeed