back when I played Pokemon Yellow for the first time, my team consisted of just my Pikachu and this guy. They were just BAMF who slay the competition =D I still always have a Vaporeon in my games, even after more than a decade.
The 3 originals, Jolteon, Vaporeon and Flareon were my favorite evolutions from Eevee. My favorite of the three though, is Vaporeon since I always liked using Water Pokemon and due to the fact that Vaporeon could turn invisible.
I probably would go with THE UPCOMING GHOST-TYPE EEVEELUTION!!!
But really, my favorite design wise is umbreon.
battle wise, flareon. it can learn some good fire moves through a glitch. well not really a glitch, more of an overlooked sequence.
I don't use many ice-types other than Glaceon too...
I had a Eevee moment where I evolved a bunch of Eevee into thier other forms... heh heh... I had an army of Eeveelutions
But my favourite has to be Flareon, I like her design, I like fire-types overall, Shes strong for an average pokemon, and in PKMN stadium she shakes her butt to her opponent before battle.
Second would be tied with Espeon and Glaceon
Before 4th gen, My favorite was Espeon. Now my 2 favorite are Glaceon & Umbreon. There arent alot of ice types compared to the other types eevee evolves into, so I like Glaceon cuz of it being a elegant-looking ice type that is pretty powerful too. I like Umbreon cuz of The cool-looking yellow rings & the mysterious look of it. It has a decent selection of dark type moves & an extension of other types of moves too. It looks even better when you happen to see a shiny one with glowing blue rings!
I personally believe Umbreon is the best Eeveelution. It has an awesome moveset, and it looks just plain cool. It also fits me the most compared to the Eeveelutions. Umbreon is the best one from all of them in my opinion. Umbreon also has those golden rings around its body which is a very epic quality to me.