What's the very best word you could ever find in any of your reviews? Stop trying to think of the most fantastic superlatives you can muster, because the best word you can find isn't a synonym for "fantastic." It's believable...and you can write books that are even if the pages are filled with unicorns and dragons. Let me tell you how.
Believable writing isn't about creating a book that takes place in Dayton, Ohio. It's not even about writing stuff that's actually true (or even stuff that seems like it could be). Your book can still have believability even if it takes place on the planet of Jupiter (however, since Jupiter is a gas giant this would take some real creativity). Believable writing isn't the setting, and it's not about whether or not unicorns and dragons factor into the story. The Harry Potter books, at first blush, are ludicrously unbelievable. But read them, and you're going to catch yourself wishing you could get an Owl and an invite to go to Hogwarts.
Because believable writing is in the characters you create. That's where believability lives and breathes, that's where it comes to the page. If you build a character that your readers can relate to, then they can also believe in fantastical things like unicorns, and magic wands, and vampires that live in a tiny Washington town. Anything happening around the character is believable if readers are buying into your character.
So what you really need to know is the secret of creating believable characters. This is one of the most difficult things to do as a writer, and yet it can be summed up in just one word: reacting. The way your character reacts to external events and their own internal thoughts is what makes them feel real...or unreal, as the case may be.
A lot of authors have difficulty writing reactions (at least, that's what I've observed). It's not easy to do, particularly if your story does involve fantastical elements or situations that you yourself have never been faced with. Lots of authors write stories involving vampires, for example, and I am completely certain that none of them have ever actually met a vampire. Yet the good authors can make even these horror creatures seem believable if the characters are reacting in ways that feel real.
The trick to any good writing is using your own imagination. Don't skip over the reactions because you can't figure them out or you think they may not be important. Put yourself into each and every situation that your characters are in. Really see yourself there. Smell the smells. Feel the weather. And observe the events as they transpire. What would you do if this was happening to you? If you were seeing these things unfold? How would you react?
You're a real person, so chances are high that you would have a pretty normal human reaction if you were faced with something completely wild -- let's say a dragon, for example. What would you do? Scream? Freeze? Start sweating and having a severe asthma attack? Be honest with yourself about it, and write honestly. Characters that react in real ways are more believable to every audience.
This rule does get bent, a little, if your character is a crazy person. Say you're writing a book from the mindset of a psychopath killer, for instance. Obviously this character has to be written a little differently, but if you focus on putting yourself in that mind you'll be able to write it with a lot more believability. It helps that the vast majority of your readers won't be psychopath killers, and probably don't know what it's like to be inside the mind of one.
Be believable by writing characters your readers can believe in, and you'll start seeing that great word in your reviews.