If we define "individual gimmick" as a mechanical/spring-loaded feature of a Transformers toy that isn't to do with its transformation from one mode to another, then what are your nominations for best individual gimmick?
I was always impressed with Beast Wars Neo Archadis's geared pistol that rotated into his open hand when you turned his tail/shield.
Also, Movie Incinerators rotor mechanism, since it works no matter how you twist and pose his arms, is pretty impressive.
Oh, and Binaltech Hound's working-suspension wheels.
I shouldn't really mention this, as strictly speaking it's to do with the transformation, but I am rather taken by the sound effects that are activated when the Tight Shot and Flash Bang T.E.C.H. are transformed. Transforming from one mode to the other gives a transformation sound effect rising in pitch, whilst reversing the transformation lowers the pitch.
In a similar vein, I like Air Attack Optimus Primal's differing flight sounds according to attitude. But that's probably electronic rather than mechanical.
I said this before in the "Lack of gimmicks in Animated" thread:
I thought the best gimmick in the movie line was one that wasn't an Automorph feature at all. Scorponok's rotary mechanism is a superb piece of engineering. I was amazed at the way it worked whatever the attitude of the arms and to make it also work in conjunction with Blackout's rotors was brilliant.
Well, last night I opened Incinerator and was very impressed that Scorponok's mechanism had been further developed and improved for this toy. Again, Incinerator's blades ("hands") rotate whatever the attitude of the arms, but this time, the rotary force is transmitted through seven joints in the arm and shoulder assembly. A very impressive piece of engineering. (There's also a nice spring-based Automorph in his chest.)