That's what I personally feel. The Dark Knight was better in every way. It was grander in scale, far more action packed and it had a legendary performance by Heath Ledger.
Batman Begins was just another meh superhero movie.
Don't agree. Batman Begins carefully tells a story about Bruce Wayne and pulls it off very well. The Dark Knight recklessly spawns a huge story with way too many things going on, too many implausible leaps, and too much noise for its own good. TDK is a good movie, just at the bottom of my Nolan films list.
Anyone who says that either of those films is "mediocre" has bad taste or doesn't have a basic understanding of cinema. It's my personal opinion that The Dark Knight is superior, but Batman Begins is still a great movie. You'd be foolish to call either of them "meh" or "mediocre".