My favourite gimmicks would be combiners, since the combining sequences always looked awesome, and as toys, it's interesting to look at the engineering and thought that went into it to make it possible.
Favourite subgroup would be Team Bullet Train from Robots in Disguise. Their train alternate modes were different and plus, they combined into one of the best combiners ever... I was enthralled when they first appeared in RiD, and while watching the episode, I was thinking "if only they could combine...", so I was so excited when they did by the end of the episode. Their toy form were great too and one of my favourite combiner toys.
"sometimes the things you see might not be real and the things that are real you might not see"