Though I love all the Eevee Evolutions, were I to have to chose one, it would have to be my namesake; Glaceon. The top of the pile has changed hands quite a lot, for me, but with the release of Glaceon in Generation IV, I just couldn't pick anything other than Glaceon.
Ice also happens to be one of my favourite types in the game, so there were bonus points for it, there. :P