That brings up a very important question: What is the Definitive version of Batman?
Actually, we don't have to go far to see another really great interpretation of Batman that is just as valid as The Dark Knight. In 2008, the very same year that gave us Nolan's Bat-masterpiece, the Cartoon Network premiered Batman: The Brave and the Bold, a cartoon about the adventures of the Caped Crusader and various other heroes from throughout the DC Universe. While The Dark Knight was a great piece about the self contained world of Batman, Batman: The Brave and the Bold showed off how Batman can work in a big shared universe. This is a Batman who works with Aquaman, The Green Arrow, Blue Beetle, and everyone in between. Also, while The Dark Knight showed us how he can be a dark, brooding character in a very serious world, The Brave and the Bold gives us a Batman who is still a serious individual, but this time in a goofy, fun world that feels like it's sprung up right out of the Silver Age of Comics.
Both of these Batmen are very similar characters; they both have all the gadgets and they both fight crime so that no one else ever has to go through the tragedy they've endured. The difference between the two is the world they occupy. Nolan's world is grounded and serious. It's one that any adult can enjoy without having to hide it. Batman: The Brave and the Bold's world is one that your kids can enjoy and that a good deal of adults might not want to own up to enjoying. Both versions are well written and stay true to the very foundation of the character. Still, I don't think I can stop here and say that this is the definitive version of the Bat.
Let's stick to the year 2008 and take a turn to the Batman comics book. At this time Grant Morrison was still early in his ongoing Batman saga and he'd just gotten to the first big story of his tenure, Batman R.I.P. This is a story of Batman facing the ultimate evil; the leader of the Black Hand, a man claiming to be Bruce Wayne's father, Thomas Wayne. The build up to R.I.P. and the story itself draw much from Batman's long history, even using material from stories deemed to childish or fantastic to actually work in a modern Batman story.
One of the most prominent themes Morrison has run with is Batman as a concept. Batman and Robin follows the exploits of the original Robin, Dick Grayson, and Bruce's son Damian as the Dynamic Duo. Bruce Wayne is presumed dead throughout the sixteen issues of the book that Morrison penned, while the last couple of issues came out simultaneously with The Return of Bruce Wayne, which saw Bruce falling through various periods in time, each time becoming the Batman of that period. Each and every example I just noted tells the story of Batman in a different manner, but each one sticks to the key elements of the character. All of this leads into Bruce's return and the beginning of Batman Incorporated. Batman Incorporated follows Bruce as he travels to world and begins to deputize various individuals as the Batman of their region. The entire point of this run is that even Bruce Wayne isn't necessarily a defining characteristic of Batman; he can work in any setting. This is the closest of these three examples to what I could call a definitive Batman.
With that I think I've come to my point: there really is no definitive version of the character. Batman is just such a versatile character that he can be applied to almost any kind of story and situation. He can be serious, dark, and brooding. He can be a goofy, corny guy who spouts out silly one-liners. He's been portrayed by such actors as George Clooney, Adam West, and Christian Bale as well as lesser known actors like Diedrich Bader and perhaps most notably by Kevin Conroy.
Even Batman's greatest nemesis, the Joker, lacks a true definitive portrayal. Heath Ledger will certainly be remembered by many as the character, but there are many versions of the character that are just as important. Mark Hamill's portrayal of the character in various movies, tv shows, and video games is one that me and many of my peers will always remember. He's played the role for nearly two decades now, which means he's been in some of the most memorable Joker stories of all time.
Even within the comics, writers such as Alan Moore, Dennis O'Neil, Ed Brubaker, and J.M. DeMatteis have contributed to a long list great Joker Stories that shaped or analyzed the character in one way or another. Another important point of Grant Morrison's Batman run is that the Joker is a character who can be constantly reinvented. Maybe he's a psychotic mastermind who wants to best Batman or perhaps he's a crazed anarchist who wants to watch the world Burn. He could even just be a goofy criminal, simple as that. As long as that key element of the clown motif is there, then you can try anything you want.
Even the cast of characters Batman surrounds himself with is never really solid. The only two real mainstays are Alfred Pennyworth and Commissioner Gordon, and even then what role do they play? Is Alfred simply Batman's loyal servant or is he Bruce Wayne's surrogate father figure? Is Commissioner Gordon Batman's good friend or just a man who turns to caped crusader in his time of need. Maybe he has a Robin, maybe he doesn't. Who fills the role of that Robin? Dick Grayson, Tim Drake, Damian Wayne, or perhaps even Jason Todd. Is there a Batgirl? Is Catwoman an enemy, an ally, or a potential love interest? Is he a member of the Justice League and if so, how active a member is he? All of this changes over and over depending on the creative team.
The definitive Batman doesn't exist. Batman is a character who doesn't have much to define him outside of "vigilante with a bat motif." That's such a flexible idea that it really works well in just about any story you could think of. Nolan's The Dark Knight is just as valid and true to the mythos as Batman: The Brave and the Bold. As long as he's a crime fighter sworn to avenging his parent's death by striking fear into that superstitious, cowardly lot, then it's definitive. The limits of what that means lies solely in you imagination.