My favourite subgroup would have to be the Aerialbots - the Autobots finally get airborne firepower, Concorde is one of the coolest planes ever built, and they combine!

In terms of gimmicks, as a child it would have been combiners (robots that come together to form a bigger robot = awesome); as an adult collector, Automorph/MechAlive (a good showcase of the movie CGI models' complexity).