Probably the best one (and the most popular) is
FontAwesome
Because it has a huge variety of icons, in multiple styles. And now with version 6, you can add your own custom icons, and use them in the same way as their library icons. Very useful!
Grommet
Is a less popular component library. I don’t like it. I think it cuts too many corners. It does have many components, but each one is not very robust or flexible.
But, their icon library is pretty good! Great selection of very simple easy to read designs.