It's always been a common practice in Hollywood for stars and filmmakers to work on numerous productions together, acknowledging that finding that magical spark is something to be valued and used to its fullest extent. Through the years, there have been countless duos, such as and , and, and and , but one cinematic pair that doesn't earn enough recognition as a team is the work of and .

Eastwood is universally recognized as a Western icon, but his films with Siegel demonstrated his impressive ability to take on action and dramatic roles, proving him to be a versatile performer with immense range. Aside from collaborating on five films together, Siegel was also a major influence on Eastwood's career as a director and filmmaker, serving as a mentor of sorts, which essentially speaks volumes about the depth of their rare cinematic connection.

Eastwood stars in the crime thriller, , as a veteran sheriff's deputy from Arizona, Walter Coogan, who travels to New York City where he is to apprehend and extradite a fugitive, Jimmy Ringerman (Don Stroud), who is wanted for murder. When Coogan tricks his way into gaining custody of Ringerman, he's unexpectedly ambushed and the fugitive manages to escape, forcing Coogan to extend his visit in the Big Apple.

centered around an interesting correlation between the traditional ways of the Western lawman and the progressive by-the-book city slickers. The oil and water combination sets up an inevitable clash that is both thrilling and pure entertainment. Initially, Coogan's Bluff received some criticism for its excessive violence, but the majority of critics, including Roger Ebert, gave the movie a positive review and awarded it three out of four stars.

Eastwood reunites with Siegel for the second time in their only Western, , as an American mercenary, Hogan, who saves a nun, Sara (), from being attacked by a group of bandits. When Sara begs for Hogan's protection from French soldiers who have been tracking her, he agrees to take her to her destination at a Mexican camp where she had arranged to help the revolutionaries' attack on approaching French troops. During their journey, Hogan notices unbecoming behavior of the sister, who engages in drinking, gambling, and swearing like a sailor, leading him to believe that she may not be exactly who she claims to be.

Two Mules for Sister Sara is a modern tribute to the Old Hollywood Western with subtle touches of humor and comic relief that soften the intensity of the overall plot. MacLaine gives a cheeky comedic performance that balances out Eastwood's hard-nosed and sharp performance as a no-nonsense gunslinger with a heart of gold. Eastwood's performance often gets lost among his other mainstream roles, but Two Mules for Sister Sara is one of Eastwood's most underrated movies, and a performance that is more than substantial and wildly entertaining.

During the American Civil War, a severely wounded Union soldier, John McBurney (Eastwood), is found in Mississippi by a 12-year-old girl, Amy (), who takes him back to her private all-girls school. Initially, the head mistress () wants to turn him over to Confederate troops but then decides to first nurse him back to health. While McBurney is locked away from the girls in the music room, the teacher, Edwina (), and a 17-year-old student, Carol (), both begin to grow fond of the school's unexpected guest.

The third collaboration between Siegel and Eastwood, is a torrid war and Southern Gothic psychological thriller based on the 1966 novel written by The film marked Eastwood's first attempt to take on an against-type role, resulting in one of the actor's most vilely complex performances that has all the makings of an unsettling Dr. Jekyll and Mr. Hyde transformation. While the movie was a monumental milestone for Eastwood's on-screen career,

was the final collaboration between Siegel and Eastwood and is a riveting prison drama that centers around an inmate, Frank Morris, who, after successfully escaping from several prisons, is sent to the high-security prison, Alcatraz, which is located on a remote island, to serve out the remainder of his life sentence. Once Morris arrives, he immediately notices the poor living conditions and treatment the inmates are subjected to, as well as the cruel Warden ( who is the worst of the worst. When Morris notices that one of the walls in a fellow inmate's cell is deteriorating, the realization ultimately leads to Morris plotting an escape for the ages.

Escape from Alcatraz is based on the 1963 novel written by, which tells the remarkable true story of Morris who, along with several other prisoners, managed to escape from the highly secure Alcatraz in 1962. . Siegel gives audiences an uncensored view of the gritty darkness of life behind bars, capturing the emotional depth and sincerity of various prisoners and portraying them in a genuine, humanizing light. Escape from Alcatraz was both a critical and commercial success and went on to become one of the highest-grossing films of the year.

Eastwood stars in the iconic action crime film, , as the unorthodox San Francisco detective, Harry Callahan, who, after rescuing a little girl from a dangerous psychopath, Charles Davis (Andy Robinson), becomes enraged after the department is forced to let Davis go after he claimed that Callahan violated his civil rights. Shortly after being released, Davis, who calls himself Scorpio, decides to hijack a school bus full of children, forcing Callahan to stop Scorpio on his own unethical terms.

Dirty Harry is not only one of Eastwood's most iconic roles but is also, without question, the best Eastwood and Siegel film. The movie is based on the notorious Zodiac Killer, who terrorized the Bay Area during the 1960s and today, is one of the most famous unsolved cases in American history. Eastwood's character was loosely based off of one of the case's lead detectives, Dave Toschi, who had a reputation for bending the rules. Eastwood is an absolute force as a calm but deadly lawman who, at an indecent time when criminals run rampant and slip through legal loopholes, plays by his own unconventional rules all in the pursuit of justice. While all Eastwood and Siegel films are worthy contributions to classic cinema, Dirty Harry ultimately reigns as the duo's all-time best and is an absolute must-see Siegel and Eastwood production.

NEXT: The 100 Best Movies of All Time, Ranked