Go is self-hosting (https://en.wikipedia.org/wiki/Self-hosting_(compilers)) and not that old, so the source of the go compiler and its tools (https://go.googlesource.com/go) should be good examples (possibly not of all language features)
What do you mean by best?
I'm not a Go expert so hard to judge, but:
github.com/rclone/rclone - Somewhat complex with abstractions, plugins, filesystems, networking, multi-platform support etc
https://github.com/hashicorp/ has a lot of Go