Flareon could use it against Salazzle, possibly and the many Fire-Fighting Pokémon
Leafeon could use it against Breloom
I suggest looking at all the Fighting and Poison Pokémon and looking to see which ones share types with the Eeveelutions as well (aside from Umbreon).
Also, pay attention to which ones are Special-Attackers, otherwise Synchronoise won’t be as effective.