Favourite theme/gimmick/concept of Transformers

[griffin]

Over the 30 years of Transformers, what has been the best or favourite, or most recurring marketable gimmicks of the toyline?

Things like (for example - you can add your own of course)...

Combiners (groups or bonus combining gimmick between toys),
Fembots,
Beasts (robotic or organic),
Dinobots,
Leadership characters that get multiple toys (including Movie Bumblebee),
Playsets/citybases/trailers,
Homage toys (based on a pre-existing toy),
Masters (head, target, power, breast, etc),
Micro-size (including Micromasters, Microns, Cyberverse),
Giant-size (including toys/characters that don't need to be giant-size),
Cassettes and their "masters",
Multi-mode (more than two modes),
Seeker jets,
Constructicons,
Insecticons,
Pretenders,
Non-vehicle, non-beast devices (things like guns, microscope, cassette-recorder, USB stick, shoe, iceblock, hat, toaster, pen, Playstation),
Junior-demographic play gimmicks (like One-step, Bash-bots, Titans, Cyber-Stompers),
Adult-demographic (like Masterpiece, Platinum Editions, SDCC, BotCon),
Crossovers,
Single-mode/Non-convertible (like Actionmasters, Rescue Bot figurines, Titans, Dino-Sparkers, Robot Replicas, Hero Mashers, Humans)
(others - these are just some examples)


Which ones are your favourite marketing concepts and why?
Which ones have been the most successful, in general, and to the fans?
Which ones do you think have been the most recurring? (successful or not)