It really depends on what kind of prototyping you are doing. Unfortunately the prototyping scene is extremely fragmented, and there is no ‘silver bullet’ prototyping tool that will do everything. My advice would be to get comfortable with 3 or 4 tools at a minimum that all focus on a different kind of prototyping.
My process tends to include the following:
Axure — Functional UX (and conditional) prototyping. The ability to write if/else use cases makes Axure very powerful. Medium level of fidelity
Principle — UI Animation and refined transition prototyping
InVision — For really quick screen flows only
HTML/CSS — Sometimes there’s just no substitute for front end code
Proto.io — Nice combo between lightweight ux prototyping & high fidelity UI animation stuff too.
I’ve tried Framer, Pixate, Flinto, Justinmind, MarvelApp & UXPin as well … Everything has very distinct strengths and weaknesses so it’s really all about finding the tool set that will work for you :)