The short answer: NetSurf, because it is the only contemporary web browser that also works under Plan 9, is extremely resource-efficient and is not based on one of the big (= commercial) browser engines.
The long answer: It depends. I like to use eww to test the accessibility of a website, but since Mozilla destroyed everything I liked about Firefox in November 2017, I’ve been using Vivaldi as my main browser. Although Vivaldi is based on Chromium, it is quite privacy-friendly, performant and extremely customisable. Unfortunately, some websites do not work very well with NetSurf. (I like to report this as a bug to the website operator. It is fatal that everyone always assumes that everyone wants to load and execute hundreds of KiB of JavaScript).