Robin and the 7 Hoods (1964)
Despite being filmed when the Rat Pack itself was in freefall (and Peter Lawford and Joey Bishop don't even feature in this movie), the 7 Hoods is a dose of silly good fun. Based on the Robin Hood tale, but updated to Prohibition-era Chicago (of course, Frankie in green tights wouldn't be cool enough), this spoofy yarn features Sinatra as Robbo, a likeable gangster, who along with his pal Will (Sammy Davis Jr), takes on the nasty Guy Gisburne (Peter Falk). Add in a fistful of rousing musical numbers and a freewheeling tone, this is widely considered the best of the pals' films.