It all depends, just like with many things WordPress
Best for what? What do you expect the plugin to do?
When choosing a Forms plugin, I think it is important to look at the long term, especially when you are investing money in a plugin.
I think most of the people that have been working with WP for more than 5 years have used Contact Forms 7 at one point or another (or even cformsII, who still remembers that one?)
Quite soon after Gravity Forms came to the market I bought into it (with the dev license) and I have been with them ever since without looking back. At one point I have used Formidable Pro on a project for some functionality that was easier to accomplish with that plugin, but although I bought the license for the Pro version, I never use it.
On another project a client is using Caldera Forms and although I kinda like it, I still will stick with my Gravity.
It is a solid forms solution (read: it does so much more than being just a contact form) and for me it is the best!