The original Casino Royale!
Heh, I wonder how many folks even know that it exists >.<
I still come back to "You Only Live Twice" whenever I look back on Bond films. It had everything: volcano lair, man eating fish trap, ninjas, Little Nelly, and an amazing Barry soundtrack with Nancy Sinatra. It was also ridiculous in its conceit that Bond would be able to a) pass as Japanese and b) become a ninja after a few hours training.
Total popcorn flick.
The Craig Bonds are great (particularly the first one) and are picking up what Dalton tried (and failed) to do with the series. That isn't to place any marks against Dalton, but rather the producers for not going with his serious take and running with it. I do wish his (Craig's) Aston would survive longer than sixty seconds per film, though. Love, love, love those cars.
"On Her Majesty's Secret Service" deserves honorable mention, Lazenby aside...