That's actually a harder question than it appears. "Book 1" is the obvious answer -- but how do you identify Book 1? There are many series where the order of publication is substantially different from the internal chronology, so do you start with the first published, or the happens-before-the-others? (And if the latter, will you be annoyed if the author later publishes something that happens earlier still?) -- For me, I tend to prefer publication order whenever possible, even when it means huge leaps back and forth in the internal chronology. Other people argue differently.
There are also series where the relationship between entries is more complicated than simple chronology, so books may overlap in time, place, or character in a way that makes it difficult to discern the "correct" ordering. There may be no universally optimal entry point in a series of this sort, everybody just has to jump in and splash around and figure it out for themselves.
Some authors may write in several different styles or modes that you respond to differently -- you may like their humorous books, loathe their gritty procedurals-- and if they have done both in the same series, do you have to start with Book 1 of a type you dislike and slog your way through to achieve the delights of Books 3 and 5?
Does the order even matter? I've read some series where it absolutely does not -- each one is self-contained, the ongoing "arc" between installments is minimal, and there's really no reason not to treat them all like standalone books that happen to share a character or two. A variation on that type of thing: a series I have read where the books, written sporadically over the course of 25 years, all have an obviously contemporary setting at the time of writing. There were a group of purely episodic installments were set in the Swinging 60s & the 70s, but then one set in 1983 obviously came before all of those in the internal chronology of the central character's life, and a final one in 1991 -- while through all these books the central character is always the same age. Figure out the perfect ordering of a series like that and win a Major Prize!
Another problem -- does the reader have a choice? If they are browsing the library or the bookstore or a friend's collection and something perfectly enticing leaps out at them, they may go ahead and read it because that's the book that enticed them. So what if it's volume 4 of 12? Vols. 1-3 may not be there when they're browsing and may or may not be there next time either. They may not even know that vols. 1-3 exist when they start reading.
And suppose you read book 1 as a standalone, then discover many years later that behind your back it turned into a series. Does your prior reading of book 1 still count, now that you have books 2-10 in front of you? Sure, you may be able to re-read it... but if you're a person who never re-reads old books, it's book 2 or nothing for you.
