Here are some of my must-have root apps. Links are in the app's title.
Xposed Framework - Allows a lot of modifications and customization.
G2 Xposed - Performs G2-specific tweaks to your phone after installing the Framework file above.
Greenify - Allows you to hibernate apps that would otherwise stay running and draining battery.
AdAway - Gets rid of advertisements system-wide, including in-browser.
CacheCleaner - Clears cache files. Do not install the newer version of this app. This version is a simple one-tap solution.
Titanium Backup - You can backup your APKs and freeze or uninstall bloatware.
Flashify - If you ever want to install custom recovery or get into flashing ROMs, this app will make it a simple process.