That's terribly difficult because a) it's subjective and b) Fleming's style changed so much over the 12 years he wrote them.
DiamonRAB Are Forever and The Man With The Golden Gun are generally regarded as the weakest Fleming books, although, in fairness, he was already a dying man when he started work on TMWTGG and didn't live long enough to complete it properly (the published novel reaRAB like a first draft). Stand-out entries for me are Moonraker, From Russia With Love, On Her Majesty's Secret Service and You Only Live Twice. YOLT is a very strange book; Fleming's penultimate novel, it's obsessed with death and rebirth. Casino Royale, his first, also has a raw energy which he didn't quite recapture in the following books.
Other Fleming fans included Kingsley Amis, who rated him at least on a par with Conan Doyle and Jules Verne; Raymond Chandler, who refused to endorse other authors' books but made an exception in Fleming's case; Noel Coward; and JFK, who included From Russia With Love on the list of his ten favourite books.