I recently did a bunch of research into the best icon libraries. There are quite a few options available, so instead of telling you my favorite I will just link to the various libraries and let you decide!

Top picks:

The Noun Project:
https://thenounproject.com

Pixel Love
https://www.pixellove.com/browse/line/simple/

Nova:
https://www.webalys.com/nova/ultimate-pack.html

Orion:
https://orioniconlibrary.com/all-icons

Material:
https://material.io/resources/icons/

Streamline:
https://app.streamlineicons.com

Fontawesome:
https://fontawesome.com/icons

Glyphicons:
https://www.glyphicons.com/sets/basic/

Flaticon:
https://www.flaticon.com/