It's way too early for me to judge. Having just come out of TDKR two hours ago, my answer would obviously be TDKR. But the movie's still got me in a rush. I'm still taken by the hype of it. So to answer now would be foolish.
My default answer would probably be Batman Begins, just because I've always felt that The Dark Knight was less a Batman movie and more of an urban drama with Gotham characters in it. The first one had more of a style to it that I've always admired, despite both being incredibly well made and well done movies. When I've allowed time to pass, I can properly judge how TDKR fits into that equation.
To The Dark Knight Rises' credit, though, I'll at least say that it establishes three movies that are connected, yet all feel different. And that's in a good way. They all feel like chapters of a story, yet are all undeniably Batman. Props to the cast and crew for keeping it fresh for all three outings.