I've been trying out a ton of different prototyping apps, and I've settled on Adobe Edge Animate for any interaction that isn't simply linking two pages together. It's pretty easy to learn (super easy if you have any AfterEffects experience, or really any experience w keyframing a timeline).

No pointing in creating something interactive that's not easy to share. Invision wins hands down for this. You can test on iPhone, iPad and desktop. And they are super responsive. When it comes to complex flows, that's tough, most tools would need to support some sort of state machine.

Axure hands down. You can layer interactions, use conditional logic, and create animations.

Keynote! And it's not even close!