The best “lightweight” browser is Google Chrome. Chrome has been the best browser for the past decade now. Chrome has been so far ahead that I really couldn’t imagine using the Web over the last ten years without it.
The second best browser is probably Edge. Edge now runs on the same engine as Chrome. The two browsers are similar, but Chrome does things a bit better, which still makes it seem more “lightweight.” If Chrome demands too many resources for your hardware, try Edge, as it is less demanding on resources when using the Windows operating system.
The third best browser is Firefox, which, of the three, is the only open-source browser.
Note: This post is based on what feels lightweight rather than what technically may be lightweight. I don’t know the technical aspects; I only know how it feels as a user. Often what makes something heavy isn’t the foundation itself but the add-ons, and the best add-ons are going to be available for the three aforementioned browsers because they have the most developer interest in addition to the best foundations due to immense funding opportunities. They also have a better chance of offering several extensions (whereby one of them may suit you better) than other browser alternatives.
Best of the rest: Vivaldi, Opera, Brave (though I don’t recommend using Brave as it is disrespectful toward publishers).