Aiiane is still the queen of addons.
here's my list:
ActionBarColor (instead of just the numbers coloring for range/cooldown, the whole thing turns red)
AnywhereTrainer (can see all of your trainers...anywhere. Nice for checking spec during downtime)
Curse Profiler - for making nifty signatures
DuffTimer - makes a set of reliable bars for you, your target, your defensive target, and more. Default UI bars often bug.
Equipment Sets - Saves up to 3 sets of gear, tactics, morale abilities, etc. so you can swap between sets easy (really nice for healers...or PvE/PvP sets, etc.
LibSlash - makes /commands work.
LittleBlackBook - keeps a record of who you've killed, who's killed you. kinda neat.
ObjectiveTracker - wanna know when the House of Lordrenth will open for capture? Now you'll know.
Phantom - hide any part of the UI...way more than Mythic lets you change.
RealmStatus - Shows numerical values for zones, can show all 4 tiers of all 3 pairing at once.
Show Health Percent - bakes quiche. that or shows health % for doors, rams, etc. No more guessing.
Squared - better bars for raids, scenarios, parties. I mean waaay better.
Tome Titan - want to know where to go to get the unlock for Skraven (so you get a defense against and crit against buff)....without alt-tabbing.... then look no farther.
Waaaghbar Continued - Similar to FuBar from WoW. Conveniently puts all your most-often used information in numerical format at the top of the screen.
Warhammer Scrolling Combat Text - gives you a warm floating feeling watching all those big numbers fly over your head. And the heads of the people you're making BURN! (burning sensation only applies to bright wizards and players with dry itchy skin.)