I enjoy reading Brandon Sanderson's Cosmere books (obviously, otherwise I wouldn't be here), and I especially enjoy coming up with ways that one may be able to use the magic systems within it in interesting and unique ways. All of them are good, but, Hemalurgy is of the most interest to me, due to the many, many potential applications for it, and the creative opportunities it provides (assuming, of course, that you don't use it for evil, because, quite frankly, that would just be plain evil).