Hmm.. Of the characters I can think of right now, it may be Tom Natsworthy from the Mortal Engines books. I have currently only read the first one, but he's a good character. He's very far from the most badass character I know of, but his character development is great, and Reeve really made me like the character. Hester Shaw, Tom's companion in the same book, is also a great character, but doesn't have quite as much development as Tom does. Great book, that.
I think the whole cast of Avatar: The Last Airbender is great too (the cartoon), with the sole exception of Azula's two henchmen (forgot their names, but only those two characters are uninteresting). If this thread is not about the single best fictional character, then I'd clearly say the combined cast of characters in Avatar: The Last Airbender. I love that series.