The transformation itself. A close second would be Automorph/Mech Alive, because they were rather subtle in the grand scheme of things, and added to figures rather than detracted from them. Can you name a bad Automorph figure? I mean, you could say Blackout and Brawl, but that's less the design and more bad QC. Mech Alive, there is no bad take on it, period. All other line wide gimmicks tend to be bad, and I don't like them. It leads to gimmicks being shoehorned into designs where they add nothing or actively detract from the figure, like POTP Optimal Optimus. Optimus Primal pretty much does nothing in all combined modes, and turns into a brick with a cockpit.
Hm. I was never a fan of Combiners in general outside of a few exceptions (Predaking, Devastator, Bruticus), and I never really liked the idea of turning Transformers into Robotech with the Headmaster gimmick. Kind of narrows it down a bit. Transformation isn't really a gimmick since these are Transformers, though I feel that it's absolutely important. I'll go with Mech Alive. It wouldn't really work outside of the movie lines as they really are the only ones where you can see the inner workings and whatnot. Only bad one I ever saw was the 70's Camaro Bumblebee, but otherwise it was a really interesting gimmick.
Combiner Wars. I always loved combiners. Mini-cons were interesting when well executed, but too often they became an alternative to pushing a button. And many of the Minicon teams were basically micromasters, with no real integration with anything. The Classics minicon, of course, were pure trash. But combiners? If Hasbro repainted all of CW for the next two years, I would but every single figure in quads. Except CW Prime, that was crap. But combiners... !
I am a huge combiner fan including Superlink/energize, but I have to admit that combiner gimmicks sacrifice a lot of engineering. The headmaster gimmick however seemed to free up a lot of of the design since putting the head somewhere instead of inside the chest allowed more freedom in transformation and body proportions. I collected the majority of titans return for this reason, however the base modes generally sucked for the leader classes IMO.
Every thing that the Prime Wars trilogy did. "Scramble City" Combiners and Head Masters with all that we've come to expect from modern Transformers, then wrap it up with a mash up of those and some Power Masters/ Pretenders mixed in. Oh Power of the Prime rewarded us for making to through "Repaint Wars" and Titans Return Broadside.
