I haven't experimented with too many genres. I've done fantasy, mystery, children's. But the one I most commonly write is chick-lit and so I have to say that is my favorite genre to write.
It's probably because chick-lit is my favorite genre to read. I like writing characters that girls can understand and relate too. I try to make sure that each protagonist has a unique personality, gifts, and dreams. That way, every girl can pick a favorite book and character depending on who she is the most like. That' probably why chick-lit is my favorite.