I've always found most government conspiracy theories ridiculous because either you have too many people in the know to keep it a perfect secret, or you have so few people in the know you have no authority. Like OK you're in the Illuminati and you run the world. Well... who would listen to you giving orders? No one knows about you...
For me the most plausible conspiracy theories are the ones where some of it has slipped but everyone is mostly OK with it, or at least OK enough not to freak out. Like the Snowden leak or the Panama Papers. We all pretty much could guess something like that was happening, and there'd been plenty of scattered journalism strongly hinting at it, but the whole thing hadn't quite been "a story" yet. Or like the idea that Free Masons are a secret society running everything, I don't buy that, but if you want to tell me they're a rich person's club where presidents go to eat caviar and joke about their body counts, I'd buy that. I think actual deep states function a lot like that too, you have the military leadership or whatever running things and everyone kind of knows but there hasn't been a thorough expose yet. Keeping a complete secret is just too hard, someone's going to get drunk and blab to their wife. The best conspiracies are the ones where even if a journalist runs a story on it nothing will happen.
Edit: I saw a really interesting TV special on Area 51 as a teen. Usually those are bullshit speculation "no one has ever asked if this true" types but this one actually had a lot of evidence from firsthand sources and put forth a strong hypothesis. Which is that the testing facilities out in Arizona and such are all experimental aircraft testing facilities. That much is pretty much an open secret. But they went into detail talking about how this one town had all seen low flying aircraft and lights and it really was a lot of people to be bullshitting. Then they had interviews from someone who had worked on an experimental spy aircraft that was supposed to move in all directions and be silent. I guess the idea was it could float up to people's windows and take pictures or something. IIRC they even had some video of the craft in question, it was almost dalek shaped and clearly far too large to be an effective spy plane, in hindsight its probably a precursor to smaller modern drones. The show also went on to talk about how a lot of the more plausible UFO citings were probably of an early high altitude spy plane developed in those same facilities during the cold war.