Jolteon! There are a lot of gen I Pokemon I really love, but Jolteon is by far my favorite. :) When I was a kid though, Eevee was my favorite and my favorite Eeveelution was Flareon. I don't really know when or why I started to prefer Jolteon, but it just...happened, haha.
Some other favorites include Charizard (shush), Venonat, Parasect, Ditto, Chansey, Articuno...maybe it's easier to just tell you what I don't like, now that I think about it.