That Tampa Bay won the most recent Super Bowl?
No, I’ve seen that theory. Not the Democrats (because nobody saw George McGovern as a secret criminal mastermind) but the CIA.
Ironically, this conspiracy theory originated with Nixon himself. When the Nixon administration needed to stop the initial investigation, they made up the cover story that the break in was a covert CIA mission directed against some foreign enemy and asked the FBI to halt its investigation in the interest of national security.
This story eventually fell apart but some people got the idea of it being a CIA mission lodged in their brain. There would later be a conspiracy theory that the CIA had decided to get rid of Nixon for unspecified reasons so they staged the Watergate break in and fabricated evidence that made it look like Nixon had done it.
Only because the NFL wanted a dramatic comeback story and told the refs to give Tom Brady the win.
So Nixon said the CIA did it and the CIA said Nixon did it. That there’s some miiighty fiiine CT fuel.
And the Masons, who were really behind the whole thing, just sat back and laughed.
Which was exactly as the Illuminati had arranged for the Masons to believe. Useful dupes with silly aprons they called them. 
You do realize that it’s actually cats that were behind it all, right?
I don’t think there is one, since many CTs are a reflection of culture and culture varies.
I guess anti-semitism is quite popular in christian and muslim nations. But even then, that means that its probably not common in much of asia.
I know what you are trying to do here. I do think 9/11 was almost certainly an inside job, or at least allowed to happen. Pearl Harbor was no doubt allowed to happen.
To answer the OP’s question, how about the Tuskegee Syphilis Experiment. Lots of black people won’t get the Covid vaccine because of this. That was considered CT for years until it came out that it was true.
There’s a conspiracy theory that nuclear weapons are physically impossible and that the whole concept is a big hoax.
Are you all trying to convince me that my friends and lectures are more sensible than the average? Does this also count as a conspiracy? And do you believe it or are you doing it just for lulz? 
Since Watergate was an actual conspiracy, should the reaction from the diligent conspiracy theorist be to say “See, I told you this kind of stuff happens”, or to claim that it didn’t happen, that there was a conspiracy to frame the alleged conspirators?
Wasn’t there a Mel Gibson movie about mysterious government agents going after a paranoid conspiracy theorist because one of his dozens of theories was right, and it all hinged on figuring out which one?
ETA: It didn’t take long to find, I guess the name of the movie was the obvious one.
You are not seriously suggesting that anyone who is not a Christian is a CT believer, are you?
How about the guilt, as well as body count, of some major serial killers? Leaving aside a few questionable investigations that have actually been reopened by authorities, does anyone question the guilt of a Ted Bundy, Gary Ridgway, or John Wayne Gacy?
Yes, I also know that some serial killers have spearheaded their own campaigns of innocence, and attracted followers, but have any outsiders, which is to say, non-relatives or close friends of the killers led campaigns that have attracted huge followings?
Or has anyone ever claimed, and gained a following, that serial killers don’t exist, and they are all government scapegoats for murders committed by the government?
And yes, I also know that there are huge conspiracy theories around the identities of some unknown killers, but that’s not what I’m talking about.
The OP asked for the (ie, single) biggest event that has not been embraced by CTers. Serial killings are pretty big events.
Those aren’t your friends, they are agents that are tasked with keeping a very close eye on you and your activities.
Generally I’ve seen it go the other way - a lot of people claim to have had near-encounters with serial killers that don’t jive with known locations or other circumstances, and accidental deaths of young people are often subject to wild speculation about being the work of vast conspiracies.
I think the amateur sleuths who spend years digging into unidentified serial killers hoping to track them down and bring them to justice often miss a key yet basic factor. That a serial killer who was active in the 1970s but has evaded justice may not have done so because they have expertly covered all tracks and hide in plain sight as ordinary law abiding citizens. But rather they evaded justice because the night of their last known murder the killer was hit by a bus on the way home and died. Family and friends attending the funeral and an obituary in the paper with no one ever knowing the secret taken to the grave.
Sounds flippant but serial killers are mortal too. Maybe another serial killer X was caught in the act by the escaping serial killer Y who himself became a victim of an unidentified murderer.
I don’t presume to understand conspiracy theorists, but it seems to me that they should be disdainful of such an incompetently-managed conspiracy. Taping doors open? Idiots. Now, take the moon landing—nobody’s blabbed in over 50 years! Those guys knew what they were doing!
I think the people trying to identify somebody like the Zodiac Killer, who committed crimes decades ago, view it more as solving a historical mystery rather than investigating an active crime case.