In what? The Bible is (last I looked) the most sold work of literature in the world. Does that mean religious tomes are the hot genre?
Besides, the process of getting a book to market is not a fast one when done right. Try to chase what's hot today, and it may well be irrelevant by the time your work is ready for publication.
I don't have publication statistics, but Romance seems to be a perennial good seller.
Here's a listing of the NY TImes' Best Seller lists:
https://www.nytimes.com/books/best-sellers/
Follow Writer's Digest