Before looking into which markets are easiest to break in to, first consider what you enjoy reading, what you enjoy writing, and what you write well. The single most important factor determining whether you succeed (and get published) is the quality of your writing. Ultimately, if you want to get published, a good story well written is key. If you’re passionate about your subject matter, that level of engagement will come through in your story, and your enthusiasm can help you get through the tough patches when you lack confidence, inspiration, or fresh ideas.
If you must look strictly at marketability, the romance genre would be the place to break in. Statistics from a 2004 Romance Writers of America study show that romance novels comprise 55 percent of paperback book sales in the United States, and 40 percent of all fiction sold is romance. (Comparatively, the mystery/thriller genre comprises 30 percent of all fiction sold, and so-called general fiction comprises about 13 percent.) Within the romance genre, contemporary romance is the most popular subgenre, with 1,438 titles (of 2,285 romance titles) published in 2004. So statistically, contemporary romance offers the greatest opportunity to break in. But again: Write what you love, and write the best book you can, and worry about publication later.