Question: Who is credited with launching the "Spaghetti Western" film genre, John Woo, or Sergio Leone?

Context articles: The Big Gundown
The Good, the Bad and the Ugly
Bullets Don't Argue
Once Upon a Time in the West
Sergio Leone
Django (1966 film)
Sukiyaki Western Django
Django the Bastard
Gian Maria Volontè
John Woo

The Big Gundown (Italian: "La resa dei conti", lit.
 "The Settling of Scores") is a 1966 Italian Spaghetti Western film, co-written by long-time Sergio Leone collaborator Sergio Donati, directed by Sergio Sollima, and starring Lee Van Cleef and Tomas Milian.
 It was originally released by Columbia Pictures in the US as a double feature with "A Time for Killing".
The Good, the Bad and the Ugly (Italian: "Il buono, il brutto, il cattivo" ,  "The good, the ugly, the bad" ) is a 1966 epic Spaghetti Western film directed by Sergio Leone and starring Clint Eastwood, Lee Van Cleef, and Eli Wallach in their respective title roles.
 Its screenplay was written by Age & Scarpelli, Luciano Vincenzoni and Leone (with additional screenplay material and dialogue provided by an uncredited Sergio Donati), based on a story by Vincenzoni and Leone.
 Director of photography Tonino Delli Colli was responsible for the film's sweeping widescreen cinematography, and Ennio Morricone composed the film's score including its main theme.
 It was an international co-production between Italy, Spain, West Germany and the United States.
Bullets Don't Argue (Italian: "Le pistole non discutono" , also known as "Guns Don't Talk" and "Pistols Don't Argue") is a 1964 Italian Spaghetti Western film directed by Mario Caiano.
 The film was produced by Jolly Film back to back with Sergio Leone's "A Fistful of Dollars", but with a more expensive budget and in anticipation of a greater success than the Leone's film, especially because at the time Rod Cameron was better known than Clint Eastwood.
Once Upon a Time in the West ( ) is a 1968 epic Spaghetti Western film co-written and directed by Sergio Leone.
 It stars Henry Fonda, cast against type, as the villain, Charles Bronson as his nemesis, Claudia Cardinale as a newly widowed homesteader, and Jason Robards as a bandit.
 The screenplay was written by Sergio Donati and Leone, from a story by Dario Argento, Bernardo Bertolucci and Leone.
 The widescreen cinematography was by Tonino Delli Colli, and the acclaimed film score was by Ennio Morricone.
Sergio Leone (] ; 3 January 1929 – 30 April 1989) was an Italian film director, producer and screenwriter, credited as the inventor of the "Spaghetti Western" genre.
Django ( , ) is a 1966 Spaghetti Western film directed and co-written by Sergio Corbucci, starring Franco Nero (in his breakthrough role) as the title character alongside Loredana Nusciak, José Bódalo, Ángel Álvarez and Eduardo Fajardo.
 The film follows a Union soldier-turned-drifter and his companion, a mixed-race prostitute, who become embroiled in a bitter, destructive feud between a Ku Klux Klan-esque gang of Confederate racists and a band of Mexican revolutionaries.
 Intended to capitalize on and rival the success of Sergio Leone's "A Fistful of Dollars", Corbucci's film is, like Leone's, considered to be a loose, unofficial adaptation of Akira Kurosawa's "Yojimbo".
Sukiyaki Western Django (スキヤキ・ウエスタン ジャンゴ , Sukiyaki Uesutan Jango ) is a 2007 English language Japanese Western film directed by Takashi Miike.
 The title of the film refers to the Japanese dish "sukiyaki", as well as Sergio Corbucci's spaghetti western film "Django."
 It also takes inspiration from the "Man with No Name" stock character variously used in the spaghetti western genre but most notably in the "Dollars" trilogy by Sergio Leone (initially inspired by Akira Kurosawa's jidaigeki film "Yojimbo").
 The film stars Hideaki Ito, Kōichi Satō, Yusuke Iseya, Masanobu Ando, Masato Sakai, Yoji Tanaka, Renji Ishibashi, Sansei Shiomi, Takaaki Ishibashi, Shun Oguri, Quentin Tarantino, Yutaka Matsushige, Yoshino Kimura, Teruyuki Kagawa and Kaori Momoi.
Django the Bastard (Italian: "Django il bastardo"), also known as The Strangers Gundown, is a 1969 Italian Spaghetti Western film directed by Sergio Garrone.
 This Gothic-themed Spaghetti Western took advantage of the success of Sergio Corbucci's film "Django", hence its title.
 A similar spaghetti western is the 1967 film "Django Kill".
Gian Maria Volontè (9 April 1933 – 6 December 1994) was an Italian actor, remembered for his outspoken left-wing leanings and fiery temper on and off-screen.
 He is perhaps most famous outside of Italy for his roles in four Spaghetti Western films: Ramon Rojo and El Indio in Sergio Leone's "A Fistful of Dollars" (1965) and "For a Few Dollars More" (1965), El Chuncho Munoz in Damiano Damiani's "A Bullet for the General" (1966) and Professor Brad Fletcher in Sergio Sollima's "Face to Face" (1967).
John Woo SBS (Ng Yu-Sum; born 1 May 1946) is a Chinese-born Hong Kong film director, writer, and producer.
 He is the owner of Lion Rock Productions.
 He is considered a major influence on the action genre, known for his highly chaotic action sequences, Mexican standoffs, and frequent use of slow motion.
 Woo has directed several notable Hong Kong action films, among them, "A Better Tomorrow" (1986), "The Killer" (1989), "Hard Boiled" (1992), and "Red Cliff" (2008/2009).