In my opinion, Stanley Kubrick is one of the greatest directors in cinematic history. He was a creative genius as well as a perfectionist.
To tell you the truth, I can't decide which one of his many great works I like best. But I'll mention a couple of those that I have been most impressed by. We can discuss more of them in the comments.
Full Metal Jacket (1987)
Let's start with Full Metal Jacket (1987). In that film, the main theme is the study of how the minds of young recruits are broken down to build them up for the purposes of the military, which is to make them killers. The first half of the film at the training center stateside details how his is done by a constant barrage of insults, stressful and demeaning treatment. It has a sexual element at a mental level. The results of this are seen in the second half of the film where the way the platoon finishes off the female sniper. Like in all of Kubrick's works, the attention to detail is remarkable. All of the acting performances are brilliant as are the dialog and the cinematography.
I should point out that military traditions differ between countries. WW II and Vietnam era US military training stuck to an old-fashioned way of thinking where brutalizing the recruits psychologically was an essential part of the training. The SS in Nazi Germany trained their recruits in a similar manner in the early days but they gave up for that philosophy because the results were poor. Germany was the first country to develop modern training methods where robotic obedience and mindless brutalization were abandoned. Instead, they focused on training the basic fighting skills needed in the field and building camaraderie and high morale. They were the first military to introduce so-called mission-type tactics (Auftragstaktik) where the idea is to give soldier an objective and the resources to accomplish it but leave it to himself to decide how to go about doing that. As a result, the German military was superior to the Allied on a unit vs. unit basis.
Some of the scenes in Full Metal Jacket were iconic.
This scene sums up what the film is all about the best:
A Clockwork Orange (1971)
A Clockwork Orange was widely misunderstood in its time. It was banned in several countries for years or even decades. In an interview, Malcolm McDowell who played the main character Alex DeLarge, a young psychopath and a violent criminal, called the film a dark comedy. I agree with him. I find parts of A Clockwork Orange highly amusing.
A Clockwork Orange is a cynical study of power. The main character represents power in its most primitive and direct form. The police (during this prison sentence his former gang members are recruited by the police after which they beat him up in a comically gleeful manner), the prison staff including the minister, the behavior modification specialists at the institute and finally the politician represent the power of organized society. The film pulls no punches whatsoever. It is completely devoid of any notions of right or wrong, laying bare the hypocrisy.
High culture and moral depravity of the worst kind are intertwined in the twisted mind of Alex. He is impossible to identify with as is every other character in the film, which is one of the reasons it must have been difficult to watch for a lot of people when it came out. This is what Kubrick fully intended. A Clockwork Orange as utterly irreverent, dark humor is the way its creators intended it to be seen.
Here's Alex undergoing the Ludovico treatment to make him viscerally detest violence and cruelty:
Wendy Carlos' music works perfectly here. It's her version of the fourth movement of Beethoven's ninth symphony.
P.S. Bonus points for the creative visuals and inventing a whole new slang by combining Cockey rhyming slang and Russian vocabulary.