Which Eevee evolution is the rarest on Pokemon?

Umbreon

Improvements

It depend on what you think, I personally think that Espeon and Umbreon is the rarest evolution, because it takes you forever to get them, you need to raise their happiness to 220 ( if you don't know what it means, it's the happiness point) at day ( Espeon) or night (Umbreon ) then level up, but things change in Pokemon Heartgold/Soulsilver, Glaceon and Leafeon are the rarest in those games, because you can't get them at all! the only way you can get them is trading it into Diamond/Pearl/Platinum, evolve it, then trade back. You have to find a water/fire/thunder stone to evolve to Vaporeon, Flareon and Jolteon, and I can't find those stones!

So it really depends.