Irene was never a huge focus in the books, and appeared only in one story. Even Moriarty, dangerous as he was, I felt we saw little of his intellect on page.
I think people like to play on a possible romantic connection between Irene Adler and Sherlock, whereas I feel that was probably not the point of her character. It was probably to show Sherlock’s attitude towards women and show her as a smart and capable as well as independent woman who earned his respect in one brief encounter, but that was it. As a contrast to the general view of women at that time, she showed that women can do it too.
One of the best antagonists for me is John Clay-that guy in the Red-Headed League. It’s one of my favorite tales in the entire canon, possibly my favorite.