1. Oswald acted alone and Ruby acted alone. (I support this.)
Pros - It's easy for there to be two crazy people out there in a nation of over 100 million. The real crazy idea is why did JFK ride in a convertible? In a 1000+ page book, Bugliosi lays out the evidence to support this.
Cons - It relies on there being two lone nuts who would be in position to kill the president and then enemy number one.
2. The Military Industrial Complex, CIA + Military orchestrated it.
Pros - Think Deep State national security. There were odd CIA connections with regards to Oswald and others in Oswald's circle. Dulles and the CIA were still smarting over the Bay of Pigs fiasco as well as Dulles' removal. The military was upset over the Cuban Missile Crisis and the trading of missiles in Turkey for the potential of missiles in Cuba. The military was also worried about Kennedy's commitment to Vietnam, but that is overblown by revisionist media types. The CIA has acted shady about the Kennedy assassination for decades. There is something there but for other reasons like Kennedy being a joke (we'll get to that later).
Cons - Oswald defected to the USSR and came back, which makes the FBI-CIA watching him make sense. Kennedy started a giant build up in the military, he was committed to Vietnam, and he allowed the CIA to operate freely in the 3rd world, killing whomever they wanted. Kennedy signing off on Diem's assassination was a nod to the military as it put their guys in charge of Vietnam. Even National Security is a bureaucratic process so the figurehead may not be as critical.
3. The Mafia did it.
Pros - Mafiosos were angry about Kennedy's betrayal after they helped him win in '60. Mafia guys joked they killed the wrong Kennedy a couple years later. Ruby was a low level mob guy. If they did fear for their entire system's future, they would have motivation to take a president out.
Cons - In all of the years of Mafia trials, no one rolled on the Mafia boss who called that shot. The Mafia also was not a sophisticated machine like the movies portrayed. No way do they pull off a big kill and no one let loose that nugget to avoid jail.
4. Oil interests aligned with Nixon did it.
Pros - The oil men were upset with oil tax allowance rules Kennedy was trying to alter. They also needed US imperial force projection to protect interests in the developing world. They knew that shady jerk had tons of contacts from his days as VP under Eisenhower organizing foreign coups and assassinations. If he helped them, they'd help him in the future for the prize he always dreamed of: the presidency.
Cons - Nixon was making money. After the California governor election loss, he looked like a dead figure in US politics. I like the motives, but this makes no goddamn sense considering....
5. Oil interests aligned with LBJ did it.
Pros - LBJ was a much closer tool to use in the Oval Office. Same set up as Nixon. The oil men wanted LBJ in power to protect their oil allowances. They knew dirt on LBJ, making him easier to control than the oversexed playboy in the White House. LBJ had resources and connections throughout decades in Washington as well as the underbelly of Dallas with the political machine to put into play that fateful day.
Cons - What cons? Sounds very plausible because of the oil interests, which for decades have been one of the last remaining individual sources of wealth and power outside the FDR created structure starting in the '30s. LBJ was the master of the senate, ambitious and a son of a bitch.
6. Anti-Castro Cubans did it.
Pros - Kennedy betrayed the Cubans at the Bay of Pigs, and then gave up on regime change after the missile crisis.
Cons - Not a lot of contact between Cubans and the figures in Texas unless you bring in the CIA, and would they allow that? Cubans might be stereotyped as passionate people, but passionate enough to kill the president over the Bay of Pigs?
7. Pro-Castro Cubans in the US did it for Castro.
Pros - Kennedy had tried to remove Castro. Castro gets his revenge.
Cons - See above. Minimal contact and what was there was through CIA, who wanted to remove Castro.
8. Deep State of oil interests and intelligence community did it.
Pros - This is the theory from Russ Baker's book on the Bush family "Family of Secrets". Dulles connects figures (some Cubans leftover from Bay of Pigs), as does Poppa Bush. It sounds plausible that the nexus of military intelligence and oil companies with international exposure wanted to remove Kennedy and had no figure to run against him in '64, so they would kill him and install LBJ who was more under the thumb of Texas' oil money.
Cons - Sounds plausible. Baker does a great job spinning the yarn.
9. The Soviets did it.
Pros - Oswald's weird time in Russia and even his Russian wife.
Cons - They were getting what they wanted out of the US with Kennedy alive, why would they risk relations and what they saw as an advantage by killing Kennedy?
10. MI Complex, CIA, Military, Texas oil interests, LBJ, Nixon, Anti-Castro Cubans and the Mafia did it. (My favorite to laugh at.)
Pros - I just listed every enemy of the rising left. Convenient for the left to spin webs that include every single obstacle to their progressive crusades. Combining all factions' interests, Nixon and former CIA Director Helms' own odd behavior and references to Dallas in '63 and Russ Baker's notes on the tapestry of all of these elements, it could be conceivable that they all could work together from prior exposure and common interests in removing Kennedy. LBJ invited JFK there to speak for him and Texas Democrats. Nixon, the CIA and anti-Castro Cubans went back to the Bay of Pigs and up through Watergate. Nixon infuriated CIA Director Helms when president on many things. One of the biggest pin points was his request for the Bay of Pigs file. No one would admit it, but what if this Deep State knew Kennedy was a pill popping, skirt chasing joke behind the scenes and thought him untrustworthy with the nuclear stakes suddenly at risk? People forget that it was Papa Joe Kennedy who wanted one of his boys to be Prez, and it would have been Joe Jr. if not for his death in WW2. The Bay of Pigs, Cuban Missile negotiations and Berlin would not inspire a deep state in succession. Kennedy's foreign policy was crisis management, jumping from one crisis to the next without an overlaying theme. This Deep State committed to anti-Soviet communism may have worried the GOP couldn't herd the cats together to beat him (Rockefeller may have, but the GOP rejected him), and no means of primarying JFK in '64.
Cons - How do you keep so many people quiet? How goddamn stupid would any of these elements be to trust secrecy to one of the other factions when all of these factions have competing interests outside of what they shared? Wouldn't some of the figures that went down in Watergate give information up when arrested for that? As the Mafia was picked off through the following decades, wouldn't they have used this as a chip to save their hides? A follow up to this would be wholesale killing of many people involved, which means a double conspiracy.
Riffing on all of this, what if the joke is on us, the presidency had less power in 1960 and JFK was an unstable figure at the top? While there is the American martyr issue, there is also the "if only Comrade Stalin knew" issue. JFK living would have made Vietnam, the Great Society, Urban Renewal all work, but in reality LBJ took over, and everything else about the administration remained the same. JFK would not have changed the implementation and execution of those liberal ideas. Boomers and progressives can say everything went to crap because of his death as a coping mechanism for the destruction they wrought on the nation. As a martyr, he remains the young, handsome, and glib president who pushed us to go to the moon. The media covered for years, but they could not hold back everything. Even now we are learning about JFK's seedier sex and drug issues, and it freezes people from admitting he was awful.
The conspiracy ideas above hint at the idea that what if the "Wise Men" who finally told LBJ to cut and run in Vietnam after Tet knew JFK sucked, but feared he was so wildly popular for the masses that he'd be in for the entire '60s. A lot of decisions get made in the civil service and administration before they get to the president, but the big ones eventually gets passed up to the commander in chief. If the JFK that was a reckless sex maniac with constant back pains, popping pills to stay up with his best and brightest team was the real JFK, then maybe the Deep State arranged it, and it was good for the nation that he died. Who killed JFK? His own recklessness as exhibited by a man endowed with the responsibility of the highest executive power in the greatest nation on earth choosing to ride in a convertible.