My vote goes to the first film. It’s a crucial element in setting up the Transformers as a credible threat to the world, and gives them a reason for being on Earth rather than the simple “Decepticons invade to destroy/conquer, Autobots arrive to defend the Earth” motive. TFilm1 had a great setup: the teaser trailer was in the form of a top secret transmission that foreshadowed the Transformers’ arrival on Earth. Later on, the film revealed Sector Seven, a MIB-styled organization that focused on the study of extraterrestrials (which in this continuity is Transformers-only material, since they’re the only aliens to have hit Earth so far), as having in their possession a frozen Megatron (aka Project Iceman) and the AllSpark cube (which was definitely a surprise). It stands to reason that the Transformers could not appear on Earth and be totally hidden from humanity. There must have been evidence of their existence/appearance somewhere, enough for an agency to keep track of any related occurrences. Sector Seven works well as a plot device that weaves together the search for the Allspark, the military/government angles (S7 itself as a govt. agency dealing with issues like this – they located and secured Project Iceman and the Allspark) and the Witwicky family connection (Archibald discovered Megatron, got his glasses engraved with the Allspark map), having it all culminate in Megatron getting loose, the Allspark getting evacuated to Mission City and the inevitable showdown that occurs there between Decepticons and Autobots/humans. ROTF had a great concept: Simmons was inspired by Project Iceman to search for other Transformers hidden on Earth, and came across possible identities the Seekers, the Fallen’s servants. What killed the concept was that his idea was turned down (Sector Seven had a Transformer in their own basement and nixed the possibility of others on earth? How stupid is that?), and the fact that it was only treated as a minor plot element that was brought into the film at a later stage. It could have done more plausibly: ex-agent Simmons continually tries to join NEST with his claims and evidence of past Decepticons, but is rebuffed in favour of dealing with present ones. This would have more smoothly led into the search for the Seekers, as well as provide a chance for more of them to appear (maybe Jetfire leads them into Operation Firestorm for one last fight against their former master or something). DOTM had my least favourite conspiracy theory. It was just too different from the other two films (and creates some obvious plotholes), and it served no purpose other than being the half of a long-forgotten-but-now-revived plan? Yeah, see how that goes. Besides, Decepticons don’t need spies – they ARE the spies, as robots in disguise.