Lui Extended: my favorite UI mod. Has a lot of great features.
Combat Metrics: Great way to keep track of your DPS, including your % of the group DPS, the post fight report, and the ability to post single and total damage done.
Skyshards: for gettin them skill points.
Lorebooks: for making that mages guild grind less hellish
Papa Crown: Insanely useful for raids, makes stacking a million times easier. Super low bearing too.
Potion Maker: makes alchemy less chaotic.
Srendarr: super nice buff tracker, but it is a pain in the ass to set up.
Lazy Writ Crafter: For those of us that are insanely lazy.
Don't hesitate to speak up about any addons you know that are super useful!