Firefox. It's a good browser; developer-friendly, free software, extensible (ad-blockers and privacy tools are a must), modern, and fast.<p>It is also the only browser with significant market share that exists primarily to serve its users, not its vendor. As someone who feels strongly about preserving the open nature of the internet, I find it important that a strong independent browser balances out the corporate browsers (most notably Chrome/Chromium, Safari, and Edge).
Chromium, because I'm used to Google Chrome and I like Google to keep track of everything for me. I've switched between Chrome and Chromium a few times, but I've found that Chromium works just a bit better on my system (Ubuntu 17.04).