For whatever reason, I have somewhat of an affinity for electric types. That being the case, my favorite "eeveelution" has aways been Jolteon. I honestly prefer some of the others (like Umbreon) on an aesthetic level, but something about Jolteon has cemented it as my favorite.
I dont have a favorite, but i do have a tie for my number 1 spot. lol, a three way tie.
Vaporeon- I really love the blue color in Eevee here, plus her tail looks like a mermaid! this really helps Eevee do water type attacks. And when Eevee is hit by a water type attack, it restores HP by 25%(Water Absorb thingy) .
Jolteon- i think that jaged cuts in Eevee in this form make it look bad a**. lol. Jolteon is the 8th fastest pokemon in the game....or thats what i read anyways. my favorite attack of Jolteon it the "thunder" attack. mainlycause it looks cool& does lots of damage.
Flareon- I think if i were to have Eevee as a pet, id want it in this form because of how fluffy it looks lol. Flareon is one of the three orignal Eevee evolutions, this form excels at physical attacks. my favorite attack of this one is "Scary Face" ...it packs a punch lol
Jolteon is my favorite Eevee and my favorite pkmn,he's so badass,I like his sprite in the Dia/prl games,he one of the fastest pokemon,he always starts to attack first,he's like a leader(lol) and Electric type is my favorite type.The Eevees are all my favorite pkmn and I was very disappointed that the 5th gen had no new Eevees and Espeon is my 2nd favorite.