The most effective Eeveelution is Flareon.
Its STAB attacks are super effective against Milo and Melony, it has Superpower for Marnie and Piers, and resists Kabu, while able to hit him super effectively through Dig. It has a very limited movepool though, and you're probably going to have to add some special attacks, but its Special Attack is already good. Its Shadow Ball/Bite handles Allister. It's not very good in Raihan's and Nessa's Gyms, so pairing it with a Grass Pokemon like Rillaboom is a massive advantage.
Flareon's Ability Guts is another great thing to mention about it. Guts + Facade isn't STAB, but it's more powerful than a STAB Flamethrower and it's physical, so why not.
Leafeon's high Speed, Defence, and Attack give it decent physical bulk and it can give out powerful attacks with Swords Dance. Leafeon is able to beat Melony's Lapras, and even use Last Resort if facing a strong opponent. It's good against some of Raihan's team and can sweep through Piers with X-Scissor.
Use Flareon if you choose Rillaboom as your starter. Use Leafeon if you choose Cinderace as your starter. Either one is good with Inteleon as your starter, although if you choose Inteleon I think Leafeon is a bit better, having a better movepool and better stats.
So it depends on the starter you choose :)