Best Linux Flavor

After a somewhat lengthy hiatus, I’m back. And I need your expertise.

I am leading a group that is QA’ing a new product that will is hardware with a web portal and a desktop app. Think of something like iTunes…

I know nothing about Linux. I want to include this platform, but need to know which version of Linux is a)most used and b)most popular (the two can be mutually exclusive).

And secondarily, if you use Linux, what browser do prefer? And why?

Thanks and glad to be back!