I used Lightning for a long time, but at the moment I’m pretty happy with Fennec F-Droid due to the great work of our maintainers Firefox Klar is another option to mention here.
I use Firefox with the help of FFUpdater from F-Droid which downloads the latest Firefox version corresponding to my device. I tried Fennec for a while, but had some videos issues.
Updates are not automatic, have to run FFUpdater, but nice enough so far.
Firefox v57 (Syncs Desktop ↔ Mobile and supports extensions e.g. uBlock Origin)
Lightening (If you don’t want any sync functionality) ← This browser I use when I’m not sure about what web site I’m heading to, because I hardened it, no js, no web fonts, ad block, etc… Plus it’s a smart browser, would ask you if you like to use tor.
Last thing to mention, is Orfox browser. good to have if you’re located in censored places.
Lightning is a very good browser, but by default it exposes a very detailed user agent string, making fingerprinting easy. I’d recommend to change it in the settings.
Privacy Browser is a good option for privacy-concerned users because it has safe defaults.