This is a very good question. I would have to say that in my opinion, the most underrated genre is probably the realm of fantasy. Most people I have spoken to automatically think about Tolkien's Lord Of the Rings and The Hobbit or Rowling's Harry Potter and therefore subconsciously stamp it as perhaps 'already done to the best it can be.'
The realm is vast and creation can be quite unlimited if your imagination go get to work on it and spend some time within it. There are dozens of books on the shelves which all take place in their own amazing worlds, dominated by amazing and beautiful characters of all races and species. Then there is the magic that comes with it, whether this is shot from the hands of an Elven mage or from a druid like figure practising some forgotten art.