( Not including religious books like Bible or Koran.)
I would guess in the US the answer is Catcher in the Rye. It still sells a good amount each year even now.
Might be other books in the UK, France, etc.
To the best of my knowledge, there’s never been a Foundation movie or a Neuromancer movie.
There’s been no film adaptation of A Confederacy of Dunces, but that may be coming soon.
Looking at the “top selling books” list on Wikipedia, the first one I can find is the self-help, non-fiction book Think and Grow Rich, which likely would make a poor movie.
The second is, as you mentioned, Catcher in the Rye, which has been attempted for adaptations but never actually made.
It hardly seems necessary to exclude these a priori since the best-selling ones are already excluded by virtue of films like The Bible and The Message.
I have absolutely no facts to back this up, but the Guinness Book of World of Records has sold many copies.
And it’s also the subject of several popular television series. They’re not technically movies, per the OP, but I think in spirit they ought to count.
Perhaps it would be better to restrict the question to works of fiction. I’m sure the vast majority of non-fiction books, no matter how popular, have not been made into movies.
Although not technically a movie version of the book, “Catcher has been made and remade for decades under various noms de crap, most recently as the rather slyly titled Chasing Holden (dumped straight to video, blessedly) and The Good Girl (co-starring Jake Gyllenhaal as “Holden,” a cashier in the rye), but also previously as Five Easy Pieces and the complete works, more or less, of Wes Anderson and Whit Stillman.”.
A few years ago there was talk of a Hollywood adaptation of Paulo Coelho’s The Alchemist, but it hasn’t happened yet. Judging from the Wikipedia article for the book, there hasn’t been a foreign film version either.
There hasn’t ever been a film adaptation of 100 Years of Solitude.
Can someone deconstruct Five Easy Pieces as inspired by Catcher in the Rye? I’m not getting it.
No film version of a Pratchett book unless I’m severely mistaken.
You are.
Could you link that page? I can’t seem to find it.
I was aware of the TV adaptations both live-action and animation but nothing big-screen as yet.
The list of best sellers has JD, JK CS, and JRR near the top. Guess it’s a good idea to use initials if you want to sell books. 
I’ve heard rumors about Johnny Depp doing a movie version of “The Ginger Man,” but so far there hasn’t been a film adaptation.
Heck, last year alone gave us Christian Bale as Moses in EXODUS, and Russell Crowe as Noah in, well, NOAH – and then go back through the years for Jim Caviezel as Jesus, and Victor Mature as Samson, and Anthony Quinn as Barabbas, and Gregory Peck as King David, and Charlton Heston as John the Baptist, and – shucks, how much time you got?