I've complained about the conspiracy-theory thinking that people have sloppily let pervade their political thinking, but it's also important to have a sense of proportion about these things -- you're not going to eliminate conspiracy-theory thinking (it arises from natural features of human mental and social life), and the problem with it is when you start handing it the keys to the car. I've seen a lot of people characterize QAnon as dangerous in the past few months. It probably would be if it had any real power, but in practice QAnoners seem usually to be participating in it from the fun of the social interactions, and QAnon is not an activist conspiracy theory (like Russiagate or 9/11 Trutherism sometimes are), because it is the central message of QAnon that the Satanic pedophile cult running the world is unraveling on its own.
In any case, proportion is called for, which brings me to the Anti-Masonic Party. We take political third parties for granted, but we tend not to ask what they have been. The first 'third party' -- and by any third party standards, a quite successful one, one of the most successful third parties in American history* -- was the Anti-Masonic Party. In 1826, a former Mason who had become a sharp critic of Freemasonry vanished. To this day nobody knows what happened to him, but as he vanished shortly after a bunch of Masons denounced him and a few people had tried to burn down his newspaper office**, it was very widely thought that the Masons had murdered him. Thus began the Anti-Masonic movement, out to save the world from the secret murder society that was trying to control the world behind the scenes. The Anti-Masonic Party was formed in New York in 1828 and took the political position that secret societies governing America was a violation of the principles of republican government, which you have to admit is quite true. It got a significant amount of support from Protestant churches -- the Masons were widely seen as an anti-Christian society -- and they were supported by John Quincy Adams and his supporters; Adams, who was not well supported in his own party, needed external allies, and many of his opponents were in fact Masons, most notably Andrew Jackson. The party had a successful populist message, because it took off. For a while it became the major opposition to the Democrats in New York; it spread to Pennsylvania and Vermont, each of which elected an Anti-Mason to governor in the 1830s. They often got people into state legislatures, although always as a minority -- but a stable minority party is a swing party, and they played a very significant role in a number of states. They also did moderately well with getting candidates into the House of Representatives. They eventually drifted apart, in part because the Whigs were more attractive to more people, but the migration of the Anti-Masons into the Whigs strongly imbued the Whigs with a populist strain that stood them well (for a while) against the Democrats. They also brought in some things they invented as part of their populism -- like party conventions.
Despite the problems with conspiracy-theory thinking, anti-cabal-ism -- an absolute abhorrence of the idea of someone manipulating things from behind the scene, of backroom deals, and of underhanded violence on the sly -- is an undeniable American tradition, and one that has influence well into the mainstream. (I suspect, in fact, that one reason President Trump does relatively well among some parts of the population is that absolutely nobody thinks he could manage to keep anything behind the scenes or on the sly.) If it sometimes experiences an algal bloom on the margins, that's not really surprising, either, and for the most part is not dangerous. It's not as if there's something morally problematic with being opposed to murder societies or pedophile cults; all reasonable people are. The problem here is just in the over-reading of facts, leading to an occasional over-reaction. You keep an eye on it and it usually burns itself out eventually without all that much damage -- not ideal, certainly, but manageable. Far more serious are revolutionary movements like nativism, anarchism, or Communism, which are often as conspiracy-theory-ridden but add the more serious problem of being willing to do violence for morally wrong ends.
* The Republican Party, of course, is the most successful third party in American history because it became one of the two major parties and has managed to stay there; it was the anti-slavery party that leveraged the political force of abolitionism to replace the Whigs. The American Party, also known as the Know Nothings, is another highly successful minor party, and had some decent success in Congressional elections in 1854 and 1855 on an anti-Catholic, anti-immigrant, anti-slavery populist platform. It began to fail when it lost its more moderate members to the Republicans, in part because the Republicans were more actively abolitionist. The Progressive Party, also known as the Bull Moose Party, is the party after the Republicans that has had the best showing in a Presidential election, under Theodore Roosevelt, but they never did all that well in other elections. While others have occasionally done well, my guess is that these are probably the best candidates for 'most successful third parties in American history'.
** The attempt to burn the newspaper office may seem particularly damning, but trying to burn down newspaper offices was surprisingly common in the nineteenth century and makes an interesting history of its own. People setting up newspaper offices would often fireproof them, to the extent possible, in anticipation of someone trying to burn them down over something or other printed in the newspaper.