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Espeon, hands down. Umbreon is a close second, followed by Ho-oh and Lugia (tie? I really like them both.), Skarmory (love the design), Murkrow, and my favorite starter, Totodile.
Hard to choose just one, but I do have a few that I prefer over the rest. These include: Espeon (purple psychic cat-thing ftw), Umbreon, Teddiursa, Azumarill, Chikorita's line, Lugia, Suicune, Raikou, and Murkrow.
Espeon and Umbreon would probably top the list though. X3
Feraligatr, this is mostly just because of the fun play through I had with it in which I used the tactic of running through the game with your starter Pokemon mainly. I also like Tyranitar, Ho-Oh, and Quagsire though.