Which are the most promising genres for writers who want to be sold? This question is a misnomer; in fact, in my opinion, it is suggesting the wrong approach to suggest to an aspiring author.
Before even considering the genre that one wants to be published in, a writer needs to analyse his or her own abilities and attributes. There is not one author who will sell or be sold in a genre in which he or she does have the right level of experience or ability, or one to which they are passionately attached.
To give an analogy, we all might know the practical steps needed in building house, but few of us would have the experience, knowledge or practical aptitude to complete such a task ourselves, much as we might like to try.
One must first understand ones own abilities. Some people are able to write in a non-fiction environment, because they have the expertise and the ability to pass on their knowledge in a way that is informative, and can write in a way that engages the reader. Others, although they may have the knowledge, will not have the writing attributes necessary to produce a constructive book.
Similarly, many authors may be able to write fiction. Some will have the ability to excel in the historical novel others are only capable of writing a book set within the timeframe they are living in, namely the present. There are writers whose imagination is such that science fiction suits their style and others who are more capable of writing a thriller. The same is true of romance, crime writing, humour and action novel writing.
Therefore, the task is to firstly find the genre that suits your abilities and styles. Once you have discovered that attribute, then you will have found the most promising genre in which your books are likely to be sold. In my view, any other approach would be a waste of one’s talents and time, and will invariably lead to unwanted rejections.