A Tap On The Head
Severity of traumatic brain injury is scored on three measures - the Glasgow Coma Scale measuring depth of unconsciousness, the duration of time spent unconscious, and the duration of post-traumatic amnesia (a period of time in which the person is conscious but confused and unable to form long-term memories). The most relevant here is the duration of time spent unconscious.
For a mild traumatic brain injury, unconsciousness lasts less than 30 minutes - in fact, the person may not lose consciousness at all. Even these mild head injuries can cause cognitive problems lasting up to three months after injury. In addition, repeated mild head injuries can trigger a neurodegenerative disorder known as chronic traumatic encephalopathy (in boxers, this is often described as being 'punch drunk'). Symptoms of CTE are similar to Parkinson's disease in many ways, and include motor symptoms as well as cognitive and emotional symptoms.
And most movie depictions have the person unconscious for longer than 30 minutes - after all, how much can the villain actually do in 30 minutes? If the person is unconscious more than 30 minutes, that's a moderate to severe brain injury. When a person awakens from such an injury, they typically awaken gradually, with a lengthy period of post-traumatic amnesia. Once they recover, they often continue to show lasting deficits, such as paralysis, aphasia, or other specific impairments. Personality changes are common as well.
A more realistic option would be to use a sedative, since unlike a brain injury, many sedatives can easily cause unconsciousness for several hours without lasting injury. However, chloroform, the most commonly used sedative in fiction, is actually not very good for the job - it takes at least five minutes of inhaling chloroform for a person to lose consciousness.
Probably the most realistic option is using 'date rape' drugs, injected or slipped into someone's food or drink. However, even then, the villain has to be a good judge of the person's weight, or else they might give them too small a dose (leaving them still conscious) or too large a dose (placing their life at risk). It's also important to remember that these drugs take time to take effect. Just as you don't become instantly drunk as soon as you swallow a shot glass of liqueur, a drug slipped into food or drink won't affect you right away either.
Injection is a bit faster, but still not instant. How long it takes for an injection to take effect depends on the drug and site of injection. Midazolam, an example I just happened to find data for, starts taking effect 15 minutes after an intramuscular injection, with maximum sedation 30-60 minutes after injection. If injected into a vein (which is a lot harder than an intramuscular injection), it takes more like 3 to 5 minutes. In the midst of a struggle, it's unlikely you could pull off an intravenous injection unless you're extremely lucky or skilled, so you're more likely to end up dealing with an intramuscular injection.
Incidentally, one of the most accurate depictions of chemical sedation in a movie can be seen in The Gods Must Be Crazy (a South African comedy about culture shock between a 'Bushman' from the Kalahari desert and several characters from industrialized cultures). At one point, some rebels have kidnapped a group of school children, and the protagonist is planning to dart the rebels with a tranquilizer dart his people use for hunting. I don't know what exactly the active ingredient of those darts were, but it's stated to take 15 minutes to take effect, requiring the protagonist to disguise as one of the kidnapped children and dart the rebels stealthily. Then follows a comical scene of the rebels acting drunk before they finally lose consciousness. Essentially, this is exactly how use of a sedating dart as a weapon would play out in real life.