I'd say that if there was a marathon, and assuming the marathoners had never seen any Bond (Because often the ones that have been seen shapes the marathon):
Dr. No
Goldfinger
Octopussy
The Living Daylights
GoldenEye
I left out Lazenby, who I think is really not that essential; to my mind, he's basically had the pendulum swing back, he was so wildly hated at first now he's like beloved for that reason, even though he's really pretty mediocre and his movie (On Her Majesty's Secret Service) is way too long.
I've never been a Moore fan so I picked the Moore one I think is the most "Moore-like" -- silly and weird and comedic and strange. The Living Daylights is actually one of my favorite of ALL the Bond movies, and GoldenEye is Brosnan's best I think.