My favorite was introduced in the first segment of the first game, provided you made the correct decision. Whether you chose Squirtle directly, or Charmander, you're going to run into it pretty early on.
But Gen II had lots of Pokemon I like, even if they aren't first place in my book.
Cyndaquil, Steelix, Porygon2, Umbreon, Tyranitar, Suicune, etc.