Ohhh. There's a flat-earther or chemtrailer around here, I know it.. Speak up. Those are my favorite!!
Personally, I think a lot of the government related conspiracy theories are rooted in an ounce of truth but then gorge on themselves due to lack of researchable facts. Plus, the internets...
There are different levels to conspiracy theories, imo. There are the comedic whacked out ones - Flat-earth
There are the the ones that have an ounce of truth but like you said, they get watered down by lack of verifiable evidence - D.U.M.B.s, secret societies (Bohemian Grove)
Then you have the highly plausible that people just turn a blind eye to - Global Elite, NWO, Illumaniti (Geogre Soros, The Rockefeller's, etc..)
Let's not forget that up until several years ago, the Gulf of Tonkin incident was dismissed as a crazy conspiracy theory...that is until the U.S. government confirmed it wasn't a conspiracy theory and DID happen the way "the crazy's" believed.
My favorites would be the NWO, and Pearl Harbor (we let it happen to get us all in on the war)