If there's one lesson the original 1980s Transformers cartoon taught us, it's that even if you were a member of an advanced race of transforming robots, that didn't mean you were smart. In fact, the Decepticons seemed to have no logic circuits at all (or, indeed, logic), because they seemed to believe these schemes might work.
1) Incapacitating Radio Technology
Whenever Megatron hears about a new bit of technology getting invented anywhere around the world, there's always at least a 50% chance he's going to steal it and try to use it to rule the world, no matter what the device actually does. Case in point: In "Blaster Blues," a scientist creates the "Voltronic Galaxer," which supposedly takes Earth communications, translates them, and broadcasts them into space to search for non-Transformer-based signs of life. Megatron steals is and somehow uses it to jam all radio signals, which, yes, is immensely troublesome — especially for the 1980s flight industry — but doesn't exactly put the world at risk. So flights are grounded because they can't reach the control tower and you can't get the classic rock station on your drive. Big whoops. I have no idea how Megatron was planning on achieving anything with this plan, although it didn't matter since the Autobots Blaster and Cosmos snuck into the Decepticon base and blew up the Galaxer almost immediately.
2) Starting a Drone Army
When you first hear the Decepticons' scheme in "Make Tracks," it actual sounds pretty solid at first: Recruit a bunch of human street gangs to steal cars and deliver them to the Decepticons, where they turn them into unthinking drones with the power to turn into also unthinking, but armed, robots. The problem is that these drones are basically not more than unthinking Transformers, meaning they're less effective than the Decepticons, so what chance would they possibly have against the Autobots when they kick the theoretically intelligent Decepticons every week? Second, the only real advantage these drones present is if they were used to bolster the Decepticons' numbers, but instead of combining forces Megatron sends the drones by themselves, and of course they get their asses kicked. The fact that the Decepticons have subcontracted out their entire war with the Autobots to street gangs and robots is highly telling. But anyway, since the drones are also remote controlled, it only takes the Autobots a few minutes to jam their signal and render the Decepticons' half-assed army useless.
3) Stealing a Random Box
To call whatever the hell Megatron thinks he's doing in "Hoist Goes Hollywood" a plan is being exceedingly generous, but it's clear the Decepticons thought they were achieving something when Dirge stole a mysterious black box from the Autobots, even if they don't know how the device worked. In fact, Megatron is so sure of its importance that he orders the Decepticons to destroy all footage of the device — because the Autobots have somehow fallen into a Hollywood movie production, which managed to catch the box on film — so no one will know about the box, even though the Autobots invented it and they have object permanence technology, meaning they remember things even without those things being in their direct sight. So the Decepticons spend the entire episode trying to destroy this film footage, and the Autobots go through an elaborate scheme involving fake lava in order to pretend to destroy the footage, if only so they can watch it and figure out what the hell the Decepticons want it for. When Wheeljack sees the box, he says it's junk, although he doesn't bother explaining what the hell it is. So the Decepticons never even got close to achieving… anything, really.
4) Dismembering Optimus Prime
I know on its surface "dismembering Optimus Prime" sounds like the best thing the Decepticons could ever hope to accomplish. And when you consider it was a freebie during their real plan, to turn New York Cty into its own little Cybertron — well, I'd excuse you for thinking that "City of Steel" shows the the Decepticons at the top of their game. But not hardly! Here's how they divvy up Optimus: They turn part of him into an alligator. They put his arm on top of the Empire State Building and have it shoot at things. And Megatron keeps the head functional to laugh at. So while the rest of the Decepticons are busy turning taxis into killer cars — because we know how well that's always worked out — the Autobots catch Alligator Prime, steal Optimus's head out of a storage closet, and insult Megatron until he breaks the panel controlling Optimus' arm, and which point Optimus remotely uses it to kick the Decepticons' ass. Even though the Decepticons managed to transform half of NYC, the fact is they weren't defeated by a single Autobot, they were defeated by a single Autobot's limb. That's just pitiful.
5) Building a Big-Ass Purple Griffin
In "Aerial Assault," Megatron and the Decepticons help a man named Ali depose Prince Jumal, the ruler of his vague Middle Eastern country in exchange for oil. Sure. That tracks. It's part II of this plan that makes no sense. Ali and the Decepticons start stealing airplane parts and reassembling them to make more drones, despite the fact that didn't work at all in "Make Tracks." However, to bolster these forces, Megatron has inexplicably made a giant purple robot griffin to "help," but the Autobots take care of the Decepticons, the drones and the Griffin alike. It all ends with Bruticus laying face down in an oil field and the Autobots setting it on fire. For extra confusion, it turns out Ali is Prince Jumal, who deposed himself and hung out with the Decepticons solely because he liked building airplanes? Or something.
6) Drilling to the Core of the Earth
Tired of trying to get his metal hands on oil or Energon or other energy sources from the planet's surface, Megatron decided to go geo-thermal and drill to the core of the planet in the aptly named "The Core." Of course, this is probably going to cause Earth to explode, but Megatron isn't particularly concerned, despite the fact that the entire reason he's on Earth is because he needs energy because his home planet doesn't have any. But his plan does get stupider; when the Autobots stage a commando raid to put "controller discs" on Devastator, who is the drill's guardian, Megatron let's them because he thinks he can override the discs. Of course, he never bothered to make sure this was true, and when he tries the override, Devastator freaks out, starts attacking Autobots and Decepticons, and of course breaks the drill. Which, sadly, is a much better end than Megatron was actually trying to achieve.
7) Whatever the Hell This Was
In the excruciatingly titled episode "Auto-Bop," the Decepticons do things, but I defy you to describe any of these accomplishments as "a plan." For instance, the Decepticons have inexplicably started a dance club called Dancitron. They have done this in order to use music to hypnotize the club patrons, which they have done for the sole purpose of constructing a building. Why do they want a building? No idea. Why bother to hypnotize random people into constructing said building instead of concentrating on construction workers? No clue. Why not use the resources used to enact this ridiculous Dancitron scheme to actually hire a construction company, or maybe just purchase an empty building? Note: This is one of very very few episodes where Soundwave laughs, which is all the proof I need that all the Decepticons were high during this entire scheme.