I remember always evolving my Eevee into Vaporeon when I was little, because it was my favorite then. But favorites switched as I got older. Like when I was younger I loved Charizard(Charmander), but now I'd choose Venusaur(Bulbasaur).
As for my favorite now I think the only reason I like Umbreon as much as Jolteon is because it glows and looks cuddley. >w<