Omen Addon

For all PvE Warcraft players Omen is simply a must-have. Omen threatmeter is one of the best addons in the game.
For those not familiar with Omen, or only familiar with KTM threatmeter, Omen tracks who has 'aggro' on all mobs (enemies) in PvE situations. It shows you graphically and numerically who the mob is currently going to attack, and how much dmg/healing you can do before the mob switches targets to you. It will only show this information for you and anyone else with omen or KTM installed, so in order for you to accurately calculate who is going to be the target, and when the tank will lose aggro, the entire party/raid must have this installed.
The only competition for it at all would be KTM/KLH threatmeters. I recommend Omen over these because the graphical interface is easier to use, it has options to turn on an alarm when you 'pull aggro' and it tracks each mob separately so switching targets it will still calculate accurately.
I can solidly give this addon a 10/10. It takes minimal resources, is incredibly user friendly, and prevents more raid deaths than any other addon. Every PvE player of any level should have this installed. It is hands down the single most important addon to install.