Leafeon is decent, but Grass doesn't hit very much super effectively, especially towards the endgame. Grass does nail Hapu, but so does Primarina, assuming it doesn't die.
Espeon is very solid, as it can learn moves like Psychic, Dazzling Gleam, and Shadow Ball. It's very frail, however, and the Friendship evolution can be a pain.
Umbreon is bulky, but its offensive movepool is sadly limited, and in-game is about offense, not defense. It's also going to be annoying to evolve Eevee with Friendship during the night.
Overall, I say Espeon, then Leafeon, then Umbreon, for the reasons described above.
Espeon @ anything
- Shadow Ball
- Dazzling Gleam
- Signal Beam / pretty much anything else
Hope I helped!