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Umbreon got massively nerfed in gen 5 when Baton Pass no longer kept Mean Look. That alone was pretty much the only reason it was in OU in gen 4, and it's tumbled down to UU in gen 5. Gen 6 didn't really do it any favors due to fairy being SE against dark. Espeon, Jolteon and Vaporeon were all very good in gen 5, but Espeon is outclassed by a few other psychics, Jolteon is being outsped by every Mega and Talonflame, and Vaoreon is slightly less effective due to rain as a whole being slightly less effective. Your best bet for now would probably be Sylveon.
