Honestly I would choose whatever type matchup so:
Jolteon---water types or grass types (bulbasaur, chikorita, treeko, squirtle, totidle, mud kip) (it's weak to ground)
Flareon--- grass type or electric type (bulbasaur, chikorita, treeko, pikachu)
Vaporeon--- fire type (charmander, cyndaquil, torchic)
Then if your planning on evolving it into a umbreon or espeon, there is not really any type disadvantages... So go with a pokemon that has effective move types on legandaries
Examples:
Charmander--articuno, (mini boss sort of) skarmory (especially with eve because that is a steel type)
Pikachu-- articuno, moltres, raquza (however u spell it) and somewhat zapdos, skarmory
Squirtle- moltres
Hoped this helped!