It's always been a tie between Glaceon and Umbreon for me. Until I started playing competitive. Then Umbreon started screwing with me in BW2 UU, and I decided that Glaceon being blue was so much nicer. Although, if you include Eevee in the list, I'm confused over my favourite all over again. :x
I decided to include Eevee because they all come from Eevee, so I put him in it. I know it says "Eeveelution" but what is an Eeveelution without Eevee?
Really liking Sylveon, only Eeveelution I've taken an interest in, very good design, and pokemon, both competitively and non-competitively.
Originally, it was Vaporeon. I struggled to open up with newer gens at first but now it has to be Umbreon. Reason is probably because I like the night hours. So quiet, the stars and no sun going in my eyes. It's peace for me.
My favorite is Jolteon, no particular reason. I'm a fan of it competitively, as well as how it looks.