Easily Binaltech Grimlock. Managed to Transform the damn thing from car to robot and back and back to robot once more since I got him in 2005, before finally selling him off. And then that meant he went from "Awesome! New Grimlock toy with cool sword" to "oh my god this fl'oppy poorly contrsucted piece of crap with bad fists and too small a sword!

I had issues with Binaltech Asterix Alert, but that was more due to poor mould quality than anything.