I'm on the Dinobot bandwagon for favourite sub-group. Some of my favourite combiners (Combaticons, Technobots, Tripredicus) rate a close second.

Gimmick-wise:
Combiners -
I'm a fan (but not owner) of all combiners. I dig the concept of a functional transformer then forming part of a bigger robot. Sometimes simplistically (scramble city) or with a high level of complex engineering (Tripredicus). I also like PCC which is a concept that I think has a lot of untapped potential.
Multichangers -
Triple, six, unoffical, dumb base-modes, whatever. I love TFs with more than one alt mode.

Are weapons being hidden / incorporated into an alt-mode a gimmick? Cause I'm a fan of that too.

How about fictional gimmicks of TF characters...? Cause the winner there ATMO is definitely Skywarp. In my universe he's like Nightcrawler in the X-MEN movies.