The following sections if followed, will ensure that you end up with an extremely secure system.
Install the Base Ubuntu System
https://ubuntuforums.org/showthread.php?t=1983344
The CIS benchmarks recommendations for hardening the system.
http://benchmarks.cisecurity.org/en-us/?route=downloads.benchmarks
https://www.thefanclub.co.za/how-to/how-secure-ubuntu-1604-lts-server-part-1-basics
debsums http://ubuntuforums.org/showthread.php?t=2225059
http://manpages.ubuntu.com/manpages/xenial/man1/debsums.1.html
/etc/issue
https://www.digitalocean.com/community/tutorials/7-security-measures-to-protect-your-servers