Not an easy question!
Back in the beginning, I would have said Ninetales. It's still among my top favorites. But now, I might have to say Jolteon, or Manectric.
My other top...thirty, or so...favorites, are:
Altaria, Articuno, Cacturne, Dragonair, Electrike, Exeggutor , Flareon, Gardevoir, Houndoom, Huntail, Koffing, Ludicolo, Magby, Magcargo, Mantine, Natu, Octillery, Phanpy, Porygon2, Regice, Roselia, Slugma, Suicune, Swalot, Umbreon, Vaporeon, Vileplume, Vulpix, Whismur, Xatu.
Yay for ABC order! ^^;
Anyway, I suppose I have to go with Manectric as my answer. Practically tied, though, with Jolteon and Ninetales!