Flash (Animate) is still my favourite mockup and design tool. It used to be so much more powerful in that you could export to a lot of different formats. And when your output is just some standard format, it didn't matter what the creation tool was. I could manoeuvre faster in that application than any other tool. And the bonus was that for anything you designed, it was almost trivial to animate it. I now find myself doing svg animation using it + plugins.

Adobe has constantly killed features in it but I still love it.