Well the three groups that haven't gotten any votes as the worst (as yet) are certainly the three I didn't think would because they are so cool! I just thought I had better put them to make the whole thing fair:
Targetmasters: Little Nebulan dudes who turned into guns and could attach to either the vehicle or robot was a great idea! Maybe they got it from Megatron pretty much being treated like a Targemaster in the cartoon. THe one thing I liked about the Action Masters was their targetmaster partners, very cool to have characters like Catgut & Gatoraider (seriously - didn't Michael Bay get alerted to that? Could have made a FORTUNE in cross-promotion in the movies with him in it) turn from animals into guns. And of course it made the smaller Seacons have more play value.
Combiners: Seriously - who the frag doesn't love combiners? Combaticons, Stunticons, Constructicons, Aerialbots etc etc that combine into giant hulking robots - just fricken cool man! To my mind one of the very BEST TF toy concepts!
Pallette-swapped characters: We wouldn't have Skywarp and Thundercarcker - nuff said! Though I would kinda have liked to see with those guys like they did with Ken & Ryu in Street Fighter, make them stay similar but with a little more individuality and difference in each iteration.