My ranking most faithful film adaptation to least (and what was taken from the novel) :
1. ON HER MAJESTY'S SECRET SERVICE - Some minor changes, but the bulk of the novel remains intact. Bond’s family motto is revealed and is later used for the title THE WORLD IS NOT ENOUGH.
2. FROM RUSSIA WITH LOVE - SMERSH becomes SPECTRE, a few minor changes, but very close.
3. THUNDERBALL - The addition of Fiona to the film seems to be the biggest difference. Other minor changes.
4. GOLDFINGER - Some of the fates of the characters are switched around, Goldfinger's scheme is altered (for the better), events are moved around, but most of the key scenes are there in some form.
5. DOCTOR NO - The first two thirds are fairly spot on, but Dr. No himself is a different character, both physically and in voice (but not in their hubris). The master plan and the death of the villain are also quite different. The death trap that Bond must escape from is far more elaborate in the novel.
6. CASINO ROYALE - Most of the novel is in the film, there is just plenty more added to the film, mostly action. We get Quantum (unnamed at that point) instead of SMERSH, we get Texas Hold 'Em instead of Baccarat and a far more elaborate death for Vesper. The book ends with "The Bitch is dead." We also get a female character named Solange, which is a name Fleming used in two short stories: FROM A VIEW TO A KILL and 007 IN NEW YORK, but the similarities end there as both of Fleming’s usages of the name are devoid of characterization, or any backstory to speak of.
7. THE LIVING DAYLIGHTS - The short story is pretty much the bit we get after the Title Sequence. It's compressed to one evening from three, and Sender becomes Saunders, but even most of the dialogue is intact. The rest of the movie is unrelated.
8. FOR YOUR EYES ONLY - The location of the original short story of the same name has been transplanted from Upstate New York to Greece. It's basically the same story of revenge, but I preferred in the book where it is M who requests Bond's involvement for personal reasons. Far more close is the adaptation of the short story RISICO (from the same collection), which takes up the bulk of the film. A lot of bits are added, but much of the page shows up in the film. We also get the scene from the book LIVE AND LET DIE, altered, where Bond and Solitaire (here it's that Havelock woman) are pulled over the coral. There is also the first and, to date, only cinematic appearance of the “Identicast”, from the novel GOLDFINGER.
9. LIVE AND LET DIE - The early parts of the book in New York are fairly accurate, as is Bond's relationship with Solitaire. Mr. Big is shown to be a much richer and more impressive character in the novel. Major scenes showed up in the films FOR YOUR EYES ONLY and LICENCE TO KILL, years after the film LIVE AND LET DIE omitted them.
10. YOU ONLY LIVE TWICE - The film being set before OHMSS already trips up Bond's motivation. We get Tiger Tanaka and Henderson, we get Bond turning Japanese and living with Kissy, but that's about it. The book ends with Blofeld and Bunt dead, Bond amnesiatic and Kissy with Bond's child.
11. DIAMONDS ARE FOREVER - The early scenes describing the smuggling operation, the characters of Mr. Wint and Mr. Kidd and Tiffany Case are about all. Blofeld and Willard Whyte were not in the novel.
12. THE MAN WITH THE GOLDEN GUN - EON really missed out by not using the beginning of the novel. That would have been a great Pre-Title Sequence. So we end up with Scaramanga (a far different character in his literary form), he does use a Golden Gun, but in the book it's a revolver that doesn't disassemble. He does have the same origin story (his elephant was shot when he was a boy). And we have Mary Goodnight. That's about all.
13. OCTOPUSSY - Very briefly, Octopussy recounts the favor Bond did for her father , that was basically a synopsis of the titular short story. We get an auction scene that somewhat recreates the one from the short story THE PROPERTY OF A LADY, from the same collection. The film SPECTRE contains the back story of Hannes Oberhauser, from Fleming's short story. The major change being that in SPECTRE Hannes is killed by his son Franz aka Ernst Stavro Blofeld (a character that is not named in the short story, we are simply told that Hannes Oberhauser had a family), rather than by Major Dexter Smythe (who spawned the character Octopussy in the film of the same name), as he was in the original Fleming.
14. MOONRAKER - Just the title and a villain named Hugo Drax. Much of the plot, characters and dialogue from the novel was reworked into the film DIE ANOTHER DAY, though much of it is unrecognizable in the final product. Colonel Moon/Gustav Graves', from the film, origin and plan run several parallels to those of the literary Hugo Drax. The film's Emma Frost was original going to be named Gala Brand, after the novel's main Bond girl.
15. THE SPY WHO LOVED ME - As per Fleming's request, just the title. Though the killers Sandor and Jaws are loosely based on Mugsy and Horror from the novel.
16. A VIEW TO A KILL - The short story FROM A VIEW TO A KILL takes place in Paris, there is a scene in the film in Paris.
17. QUANTUM OF SOLACE - Title only. Though the post-climax bit does bring out some of the elements of the very short story 007 IN NEW YORK.
Added note: There has yet to be a film using the title of the original Fleming short story THE HILDEBRAND RARITY, but the character name Milton Krest, as well as a form of "The Corrector", were used in LICENCE TO KILL. Furthermore, a nod is given to the title in the film SPECTRE, when M and Company meet Bond and Madeline Swan at a safe-house (an abandoned book store) we see on the front window that it had been called Hildebrand Prints & Rarities.
Added note: Though, not an original Ian Fleming novel, Kingsley Amis's (written under the pseudonym Richard Markham) 1968 continuation novel COLONEL SUN has bona fides that the later ones do not (he knew, worked with and, by some accounts, edited Fleming, it was published directly after the original run and he had written several books discussing the literary phenomena of Bond). Though the title has never been used, bits of the novel have shown up in EON films. The name of the main villain was altered and given his counterpart in the film DIE ANOTHER DAY. Here he is now North Korean, rather than Chinese, and his full name is Colonel Tan-Sun Moo (he later, through genetic therapy, takes on the anglo identity of Gustav Graves). Furthermore, in the novel, Colonel Sun's larger goal (beyond kidnapping, killing and scapegoating M and Bond), was to sabotage a Soviet led Communist summit, hoping that the UK would be found to blame. In the film, according to M, Zao, Colonel Tan-Sun Moon's henchman and apparently working according to his directives, was captured attempting to blow-up a summit meeting between the two Communist nations, China and South Korea. Also, the scene in SPECTRE where Blofeld tortures Bond is largely lifted from this novel as well. In the book, Sun tortures Bond using thin needles, rather than an automated electric drill as Blofeld does in the film. Much of that dialogue is taken directly from Amis, except that in the film it is Blofeld speaking, mainly, to Madeline Swan, whereas in the novel it is Sun talking to Bond. One of the final ending credits in SPECTRE is a thank you to the estate of Kingsley Amis.