Favourite: Combiners. When done well, it's fantastic.

I wouldn't call it a gimmick as such, but the addition of poseability as standard (Beast Wars era) - I'm showing just how long I've been into Transformers here - is still the greatest aspect that's been added to the toyline.

Least favourite: electronics. Some toys do it well (G1 Shockwave, for example, where it makes sense for his alt mode). But some it just ruins them - the soundboxes in voyager class and bigger in some Unicron Trilogy toys so hampered the Transformer itself to ruin them - and I just don't find the trade off worth it for some generic lights and sounds that don't really fit.