I'll reiterate what I've said more than once on this list - the 1986 Transformers animated movie is one of my favorite things ever. If I'm having a bad day, I'll remind myself how lucky I am to inhabit the same universe as that perfect masterpiece of 80's cross-merchandizing awesomeness. That film has it all - pathos (the death of Optimus Prime remains a seminal moment in my childhood), sweeping narrative, a blistering soundtrack, fantastic voice performances (Leonard Nimoy!), and a terrifying, implacable antagonist.
One of my mild complaints about the Transformers was always that Megatron fell just short of being a good arch-rival for Prime. Sure, he had swagger and a lot of firepower, but he lacked the chops to really match up, his Decepticons a sort of New Jersey Generals to the Globetrotting Autobots. I always felt like Prime was just sort of carrying Megatron through the late rounds, like a winning boxer putting on a show for the crowd. But Unicron...this was something else again. This guy ate planets, and not in a dainty, mannered, off-screen Galactus kind of way, but in a more visceral and violent fashion, the way my kids eat. You get the sense a lot winds up on the floor of the dining room with Unicron. Out of nowhere, he burst on the scene as a threat to all of Cybertron. Instead of taking this opportunity to join forces against a common foe, the looming spectre of Unicron serves as backdrop to an apocalyptic battle between the two Transformer factions, far bloodier (oilier?) than anything the show ever produced. There were an awful lot of casualties that day (including Prime himself, Ironhide, Ratchet, and Prowl, as Hasbro used the movie to clear the decks for a new iteration of toys).
With the Autobots in disarry and the Deceptions put to flight, Unicron inserts himself into their war, recruiting an unwilling Megatron as a catspaw to locate and destroy the one thing - the only thing - that could stand in the mighty machine's way, the Autobot Matrix of Leadership. Unicron (voiced with growling gravitas by Orson Welles, who died days after finishing his lines) remade Megatron into Galvatron (voiced with sneering quasi-sanity by an apparently thoroughly-entertained Nimoy). This sequence features some phenomenal dialogue as Unicron manhandles the overmatched Megatron (whose bargaining posture was highly dubious).
Later in the film, when Unicron tires of the insubordination and lack of production from his underlings, he makes the dramatic and impressive transformation into his robot mode, wings and all. It's about as cool as any eleven-year old could want. Even when he is eventually defeated by the Matrix in the hands of the headbanger dude from The Breakfast Club, he's awesome, hamming it up in a prolonged death scene. Welles left it on the field, baby, even as he and Unicron both proceeded on their way to oblivion.