Auction, Mail, and Inventory:
Auctionator – Easy to use and very powerful auctioning tool.
AdiBags – My personal favorite bag addon. Allows the creation of groups, automatic sorting, and much more.
Postal - Inbox and mail quality of life

Combat, Castbars, and Buffs:
BindPad – Lets you keybind spells and abilities right from the Spell Book.
Bartender - Actionbar replacement
Details! - Damage meter
kui Nameplates – Lightweight, sexy, and customizable nameplates.
oPie - Keybind an easily accessible "actionbar ring"
Quartz - Castbars
Raven – Icons or bars keep you up to speed with what’s happening. Highly customizable.
xCT+ - Lightweight, good looking alternative to MSBT. My personal favorite.


Raid Tools:
Angry Assignments – I like to think of it as sticky notes for your screen. Let’s you type out notes and send them to everyone in the group with the addon.
BigBrother – Informs you of who in the group is well fed and flasked up.
BigWigs – Reliable. Lightweight. Customizable. Personal preference, but I hate dbm.
BLTRCD - Raid Cooldown Tracker
Death Note - See who died, when and why
Grid - Raid frames
oRA3 – Powerful raid assist addon. Allows you to check cooldowns, manage guild repairs and auto-promote/assign masterlooter.
WeakAuras – The most powerful, do everything addon for tracking.


Everything Else:

Better Addon List – Loads certain addons only when needed.
Aurora - Ui Styling
AutoTurnin - Turns in quests automatically
Bazooka - Databroker
BadBoy – Filters out bullsh*t from chat and allows you to easily report spammers.
BugSack/BugGrabber – Catches all lua errors and can show or hide them.
CleanMiniMap - Mini map
Garrison Mission Manager
Fixtehtip - Tooltip
Masque - Button skinner
MasterPlan
MoveAnything - Move literally anything
oUF – Unit Frames library
oUF_Karma - Unit frames (Modified heavily)
Prat - Chat mod
Thogar Assist - Helps dodge trains
xLoot – Lootframe stylizer