Hate to say I can't answer this with one title -
Depends on what mood you catch me in.
For me, the 1-2 slot shifts between From Russia With Love and On Her Majesty's Secret Service.
3-4: Casino Royale (reboot) and/or Thunderball
Not coincidentally, I feel these four were also Fleming's best novels, and the films reflect that fact.
The more Fleming that goes into a Bond film the better it's been, the lone aberration being The Spy Who Loved Me, a good film in it's own right simply because it was so ridiculously over the top.