What sells best?

A lot of new authors have posed this question to me and it's a hard call. Thanks to Harlequin's multiple line of Romance novels, and Dorchester, romance novels have been hot sellers for a long time. But then you consider mystery authors such as James Patterson and Patricia Cornwell and their novels are always best sellers.
Then you move onto John Saul and Stephen King and here comes the best selling horror novels.
The real answer lies in the question that should be asked from the start: What genre do you feel most comfortable with? You can't always base your choice of genres on the sales of others. Some writers adapt more easily to mysteries; some family drama. Having an author write in a certain genre judging by past sales records of others is like buying a pair of shoes in the wrong size: They don't fit.
Writing should be natural and not forced. The words should flow and the plot excite. If you start writing a romance novel and are constantly stumped or you don't really feel an attachment to your plot, your characters, then perhaps you should try another genre, like mysteries for instance or horror. Don't fight the inner feelings you have to write in one genre when another is bringing in the big bucks. Mainly because, like shoes, if the fit isn't good the novel won't be good either.
It's not really one genre or another that's selling well, it's the author, his/her work, his/her plot, his/her character interactions. It's the work you produce in the end that will determine where your heart lies insofar as writing is concerned and how well your novels will do in bookstores.