If, like me, you don't want it to modify your configs files for you, and don't want to turn off the webserver everytime you renew certificates, use Webroot mode.
I updated my guide to setup Nginx + Ubuntu 16.04 + Let's encrypt (Nginx, IPv6, HTTP/2, and A+ rating at SLL Labs):
https://gist.github.com/cecilemuller/a26737699a7e70a7093d4dc...
I hope it helps those that hesitate to use Let's encrypt.