Well, since you already have the Water, Fire, Electric, and Psychic types, that rules out Vaporeon, Flareon, Jolteon, and Espeon. There remains Umbreon, Leafeon, and Glaceon.
If you do not have access to Diamond/Pearl/Platinum versions, get Umbreon (who evolves at high happiness during night).
If you DO have access to DPP versions, get either Leafeon or Glaceon, it doesn't matter which. Glaceon evolves by the Icy Rock on Route 217. Leafeon evolves in Eterna Forest (Moss Rock).
Glaceon is handy for when you go against Champion Lance. And Leafeon is just all-around handy to have. TM Dig gets you out of caves, and HM Strength is needed for Victory Road.