I'd probably say Post-Apocalyptic; the donwfall and/or destruction of a civilization and the subsequent consequences are intriguing, and the genre is usually filled to the brim with both political and philosophical questions, concepts and theories, allowing one to really examine the human condition in its extremes. Plus, the setting usually also involves a great host of creativity when it comes to the crafting of groups, nations, societies and mechanical things, which is always lovely to explore. If I had to limit myself to one genre, that'd be it.