Like nyah, favourite types? That's hard because...well, there are pokémon I like in every type. But, I do have a few favourites, namely Poison and Grass.
Grass because...well, come on; there's Cradily (Which can pwn any given Fire type by the way. xP), Victreebel, Exeggutor, and Jumpluff all at once. How could I not love it? *Huggles grass types* Not to mention that Grass types bring us cool moves like Leech Seed and Sleep Powder, and have a wide variety of statlines, all the way from fast-paced sweepers like Sceptile and Exeggutor to solid tanks like Cradily and Venusaur. Yeah, for some reason I tend to be pro-nature in games, even though I'm hardly a greenpeace fanatic or anything like that IRL. xD
Poison because, as certain others have mentioned, they have really neat colouring, and I don't think that there's a single Poison type which I haven't liked (Although to be honest, Dustox doesn't appeal as much to me as the others) and there are some really cute ones like Victreebel, Koffing, Weedle, Tentacruel, Haunter, Zubat, and Muk there, so yeah. Besides, poison types have a tendency to be great physical walls while still enjoying fairly high Attack stats, so I like them from a strategic point of view. Oh, and poisoning opponents is fun. >D
So yeah, those are my favourites. I'd have said Water because there are lots of pokémon in that type that I like too, not to mention their wide usefulness from a strategic point of view, but it's such an incredibly wide and, in my opinion, over-represented type that there are also several mellow, non-descript filler pokémon among them which I would rather have seen left out, and that kind of spoils it for me. ~_~ Ice is nice too, but when you have to trudge home through it every day and it gives you colds and all that loveliness it kind of looses part of its charm. xD