Well, I personally like every type, but I guess there's a bonus if the Pokemon is fire, dark, or ghost. Usually, it depends on what it looks like, and sometimes what it evolves from. Cuteness/coolness is a factor, too, and I usually like Pokemon that no one else remembers. Octillery, Nidorina, Parasect, and Unown, just to name a few. Now that I think about it, there are very few Pokemon that I don't like. -shrug- Guess I'm a fan of at least 490 of them. xD
Edit; I tend to go for female starters and female Eevee a lot, too. Why is the ratio so low? It's ridiculous, but I still manage to snag me a gal pal.