The truth is that there is no best tool of all, but rather the right tool for you, the type of project, the stage of the project, and your needs.
Before choosing the best tool for you, it is important that you ask yourself a few questions. Who knows, you may discover a better option that will greatly help your work:
What is the learning curve?
How long would it take you to learn to tinker with this tool? How long would it take you to create a prototype? Avoid picking heavy tools with many functions you will not even use. If you're spending more time trying to figure out the program rather than putting together the prototype, there's something wrong.
What kind of interface are you prototyping?