Adult commercial fiction: thrillers are a favorite. Crime of all kinds. I read widely but rep narrowly. You're welcome to query me for anything. The worst you'll hear is no.
Adult non-fiction: History and biography. Narrative popular science and math. I love maps and geography too.
Projects I'd kill for: a narrative history of the second feminist wave, much like Taylor Branch's seminal work on the Civil Rights movement
Juvenile: picture books and middle grade non-fiction. History and biography but again, you're welcome to query for anything.
Bottom line: I've taken on and sold books that don't fall into any of these categories. I read the query and was interested in the book.
I'd rather hear from you than not.
I'm more concerned that you send me what I need to evaluate your work, than I am the topic area.
What I need to evaluate your work is:
a query letter and the first five pages (or so) of the manuscript in the email (not attached) for fiction
a query letter and the overview to the proposal for non-fiction (also in the email, not attached)
There are a lot of ways to shoot yourself in the foot in a query. Mostly those happen if you're not trying or you don't care. I'm not looking to reject work. Follow these guidelines to your best ability and you'll be fine.