Office UI Fabric icons
https://uifabricicons.azurewebsites.net/
I like this library due to its very sharp edges and mostly uniform sizing across all the icons.
The list of available icons is quite extensive, but not every icon looks good together.
Feather Icons
https://feathericons.com/
This library has nice soft rounded icons, and they all seem to follow the same exact design philosophy. Every icon looks good together, but the amount of icons is 287.
Font Awesome
https://fontawesome.com/search?m=free&o=r
Even if you aren't aware of icon libraries, odds are you have seen theses icons before. This was one of the original font libraries and are still going strong to this day.
I personally have been venturing away from the use of this library over the last few years due to new options popping up pretty much everywhere.
The icons don't always match up with eachouther, but my expereince has only been with the free version, perhaps the pro version has a better selection.