My favourite would be Armada Starscream, while he wasn't the best toy, it was good enough and a lot of fun. I remember that he was the one I had always wanted to get but he disappeared from shelves before I could .

Sideways is an interesting one, with the head gimmick and I guess Armada Prime with Jetfire is pretty cool with the combiner gimmick (but not as well done as RiD).

On the whole, I think that while the gimmicks did compromise the toys a lot, they were quite fun to play with (yes, even Hot Shot with his axle-zooker). Plus, with a minicon in every toy, the figures felt like they have better value.