I really like the original design of Eevee the most. A lot of my favorite Pokemon tend to be the unevolved, cuter forms (Fennekin < Delphox all day, every day), and Eevee is no exception. A close second would have to be Jolteon, though. Then Umbreon. In my first ever playthrough of Red as a child I ended up getting a Jolteon by chance, because I didn't know how stone evolution worked, so Jolteon's always been pretty cool to me. And then you have the choice in XD Gale of Darkness to get an Umbreon or Espeon, among other Eeveelutions, and I chose Umbreon. I came to rely on that Umbreon throughout the entire game.