Probably the best prototyping app with its own great ecosystem of useful tools and resources.
Loading time & sync of data could be improved (will be in V7 though)
I love everything about InVision.
Since I started using it 3-4 years ago they improved so many things and added really useful features. They collaborate with so many top designers of the industry that they always know what we want. I really appreciate the effort, time and love they put into their products and it's always fun to use them.
The tools I use on a weekly basis are:
InVision (Webapp); Freehand; Craft; Designbetter.co