If you want to throw down the guantlet, let's start with why you believe it is not.
Why would we proceed from the default position that The Dark Knight is a masterpiece? Why are you so insistent that it is but so unwilling to address why?
I do think The Dark Knight is a flawed film, although fantastically made and hugely entertaining. There are two major structural faults: Batman leaving the Joker at the party and the Joker's ultimate fate, which is literally up in the air. This would be fine as a thematic gesture but it's clearly not - we're provided with a completely different climax and in any case we've already seen the police are in the Joker's custody rather than the other way around. Batman's ultimate fate is a needless sacrifice; the script doesn't actually require him to take on Harvey's sins as it would be child's play to cover those up as there are no witnesses anyway. The entire chase sequence is predicated on a ridiculously poor editing decision that shows the driver had no reason to go underground in the first place. And for all the claims of stunning acting, the performance of Batman himself has become the butt of jokes and the ferry scenes feature support acting that belongs on a budget TV show. We won't even get into the vexing issue of the role of women in the Nolan Batman universe.
None of these things detract from the overall presentation of The Dark Knight as popular entertainment but do mitigate against it being an actual masterpiece. Most egregious is the refusal to map the film's basic theme to its logical conclusion; Batman is not shaped or influenced by what the Joker and Two-Face represent. In the end it's a typical superhero runaround presented as something more.
What superhero movie from an all around production standpoint is better and why? What superhero movie deals with it's subject matter as maturely as TDK? What superhero movie has better acting? What superhero movie has a better script? What superhero movie comes closer as a film to being better executed than TDK?
Actually I think several superhero films meet these criteria. Many of the bigger films are equally well made and feature strong performances and interesting scripts. So there must be something else at work in the "masterpiece" stakes. You yourself indicated The Dark Knight would do for film what The Dark Knights Returns and Watchmen did for comics. I'm interested to know what you think that thing is and why.
As far as dealing with its subject matter in a mature manner, I actually believe this is one of the few weaknesses with The Dark Knight. It suffers from the juvenile "gritty = mature" mentality, or at least some of its proponents seem to. For example I think X2 actually presents more complex and relevant themes and addresses them in a more adult manner than The Dark Knight does, ultimately positioning it as the more mature picture. I wouldn't hold it up as a masterpiece, however.