I could easily say I love them all - being more of a fire fox fan (the animal, not the browser) I'd go with Flareon as my favorite. Jolteon has been getting higher on my favorites.
Flareon 1st, Umbreon 2nd, Sylveon 3rd, Jolteon 4th
going to change to a Jolteon avatar next.
They are great - love them all
As for themes, they will be great too
I'm mostly using Eeveelutions for the rest of the year, with an occasional fox in between