Mine goes without saying; Glaceon is just an awesome Eeveelution. Along with Vaporeon, I kind of put them as the most elegant of the Eeveelutions because of their types. Though generally, Glaceon is my favourite Eeveelution because it is both my favourite type and is the most appealing Eeveelution to me, for how it looks.
By Generation, it would go Vaporeon for Generation I, Espeon for Generation II and naturally, Glaceon for Generation IV.