I am a bit of an addon junkie, but even so I learn of new ones from the guys here all the time. Here is a list of ones I would recommend

QuestHelper (all)
Lightheaded (all if different classes)
Cartographer (main)
Omen (all)
Xloot (main)
WeaponSwap (I have this everywhere because I love it )
Titan (main)
Pitbull (main)
Trinity Bars (all)

Check out the wiki for a growing collection of useful addons