There's some packages that I've never ever used ... I'd say any of those!
A good example of this is anything below https://golang.org/pkg/container/.
On the other hand the net package is incredibly well designed!
The context package is also one of my favorites, not because of how it's used but rather because of how it's implemented.
I made a video on that, btw!
https://www.youtube.com/watch?v=8M90t0KvEDY