I've been missing several months of this meme, due to the lack of time and idea. But strangely (or not), an idea just crossed my mind the moment I read the theme for this month.
Sorry, it's not PnP. I know the novel has been very popular, and its first line most conspicuous that it's very hard to miss. I've been re-reading some parts of it lately and watched the film as well, but..
It's Sherlock Holmes. The first short-story collection entitled The Adventures of Sherlock Holmes begins with "Scandal in Bohemia". The story itself begins with this line:
"To Sherlock Holmes she is always the woman."
It's unforgettable because I read it 10 years ago, in my school library, imagining that Sherlock's heart is impenetrable - if he has a heart. Even with "the woman", Irene Adler, it is still debatable whether it is Sherlock's heart, or mind, or pride that puts her so high in his esteem.
There. That's my favourite opening sentence, or at least the one I remember the most.