The best Linux commands are the simplest ones. One that you use a lot is the sudo command. This is controlled by the /et/sudoers file that controls the user accounts that are allowed to access the sudo command. You can set this up so that only certain users can access sudo. This is a good way to manage security on a Linux machine.
This posting: http://www.securitronlinux.com/bejiitaswrath/useful-linux-tips-and-how-to-edit-the-etcsudoers-file-without-error/, describes how to add a user to the sudoers file so that they can run commands with elevated privileges.
Here is another posting that describes how to add your user to the /etc/sudoers file on Debian: http://www.securitronlinux.com/uncategorized/adding-your-user-to-the-sudoers-file-on-debian/.
And how to unlock the root account on Linux Mint or Ubuntu with the command-line: http://www.securitronlinux.com/bejiitaswrath/how-to-enable-the-su-command-to-access-the-root-account-in-linux-mint-13/. This is very useful if you want the proper root account back. Make sure that it has a strong password though.
Awesome Linux shell wildcard tricks. This is very useful for manipulating file listings and deleting files with strange names from your machine: http://www.securitronlinux.com/bejiitaswrath/deleting-files-on-your-linux-machine-that-have-strange-names-and-how-to-use-wildcards-on-the-shell/.
The ifconfig command may be used to set a static IP address for your server.
This is a sample of the usage of this command.
adeptus-mechanicus ~ # ifconfig eth1 192.168.2.2 netmask 255.0.0.0 broadcast 192.168.2.255
This shows that you can easily edit the network settings with the command-line and get an optimal network configuration without needing the GUI. That is why Linux is so good for setting up a network. The only thing thing that is really annoying on Ubuntu Server is configuring a Samba server to provide logins for Windows 7 machines. That is very challenging but it is possible. Supplying a network of Windows machines with logins from an Ubuntu server would be a very cool thing indeed.
This posting of mine contains some awesome shell tricks for the Linux console. Especially how to make the contents of a shell variable uppercase: http://www.securitronlinux.com/bejiitaswrath/cispa-bill-still-a-threat-and-awesome-linux-shell-tricks/.