Pure elegant transformation is always going to be my favourite gimmick as such.

however, I really appreciate good light piping, or just a well defined face (I refer again to Generations deluxe warpath which I seem to do a lot)

I did like the auto morph in some of the early movie toys (though some of them don't stand the test of time so well, my 86 Camaro BB won't stay in vehicle mode any more.) and I really liked the visual appeal of the Mech alive function in the following lines.