what a limited poll
In descending order:
The field that sparked my love for science at a really young age. Since then, I've always had an interest in rocks and the stories they tell. I used to do research for leisure, but I haven't in several years, sadly. Either way, geology will always be my favorite science by default; after all, while I probably would have wound up interested in science one way or another, it still was the one that drew me in. Special geology love...forever a special love for a special science.
This one kind of started with an interest in stars--which makes sense, I guess, since astro means "star". That includes constellations (haha), the life cycles of various types of stars, spectral shifts and whatnot. That spark of interest kind of got me into most of the other things associated with astronomy. On a side note, my favorite nebula is the Hourglass Nebula. That is all.
Study of fossils... what's not to like? Kind of a sub-category of geology, I guess--I pretty much have had an interest in fossils for almost as long as I did geology. I spent a lot of my time when I was younger looking for certain kinds of rocks, and fossils were definitely something that intrigued me. The first fossil I ever found was...what appears to be half of what was left of a nut; looked to be an acorn, but I really don't know.
It was pretty easy for me to fall in love with chemistry. Really. Having done a fair amount of research on minerals and such, an oblivious love for it was already forming when I was about eight years-old. It kind of just happened on its own because of that, but I found greater interest in chemistry-related things when I was eleven. And then I started memorizing the periodic table when I was fourteen, as most of you know. Due to the fact that I do have an abnormal infatuation with the thing, people often think that chemistry is my favorite science. It's up there, but it's not my favorite, favorite.
This also could be considered a sub-category of geology, in certain aspects, but weather science has always interested me. I think part of it was that my sister was always into bizarre weather phenomena as well, so we'd always raid the library for interesting stuffs and see who could find cooler occurrences.