A Fistful of Dollars
Although Sergio Leone's reinvention of the western lacks his follow-ups' grandeur and finessing of style, the choices he makes here are so bold and confident that his approach seems to have arrived fully-formed from nowhere. Many elements may have been happy accidents (he wasn't keen on using Morricone until he heard what he'd come up with, Eastwood was cheap and available), but some must have been quite clear (the use of extreme close-ups, the likely Bond-inspired title sequence).
Christopher Frayling suggests he invented the modern action movie, and it's certainly true that the film retains a vitality and energy while many of its peer group look dated and leaden. Several of the cast besides Eastwood returned later in the trilogy; Gian Maria Volonte in For a Few Dollars More, while Benito Stefanelli, Aldo Sambrell and chunky Mario Brega appeared in all three films.
I've always been fine with the dubbing, with one notable exception. Marisol's child, who just goes on and on and on. In the opening scene of the script, it's revealed that "Joe" is a confederate soldier who dons the poncho to conceal his identity.