World of Warcraft WOTLK Classic UI
Blizzard's stock user interface leaves a lot to be desired, especially if you are raiding. Luckily there are addon's to do everything from move or style the action bars, to showing pertinent information, or even dynamically controlling the camera. If you combine these addon's with effective key binding and a few macro's you'll be able to focus more on gameplay and mechanics.
ElvUI
ElvUI is easily one of the easiest and most complete UI addon's out there. It will remove the Blizzard frames, reconfigure action bars, name plates, and pretty much everything else.
WeakAuras
WeakAuras is a powerful and flexible framework that allows the display of highly customizable graphics on World of Warcraft's user interface to indicate buffs, debuffs, and other relevant information.
Deadly Boss Mods
The ultimate encounter helper to give you fight info that's easy to process at a glance. DBM aims to focus on what's happening to you, and what YOU need to do about it.
GTFO
GTFO provides an audible alert when you're standing in something you're not supposed to be standing in.
Plater Nameplates
Plater is a nameplate addon with a extraordinary amount of settings, out of the box debuff tracking, threat coloring, support for scripting similar to WeakAuras and wago.io + the WeakAuras-Companion for Mod/Script/Profile updates.
OPie
OPie is a radial action-binding addon: it lets you group actions into rings which appear when you hold down a keyboard or mouse binding. When you release the binding, OPie will perform an action based on where your mouse cursor is.
Hekili
Hekili is a priority helper addon that can make recommendations several steps into the future. It works by collecting information about the game's current state, using a SimulationCraft-like action priority list (APL) to recommend the next ability to use, then modifying the game state and making the next prediction.
Adibags
AdiBags is a World of Warcraft addon that displays the contents of your bags in single view, distributed into several sections using smart filters. It is heavily inspired by Nargiddley's Baggins.
Questie
Quest helper and map POI's.
Alts
Alts is an addon that allows you to setup main-alt relationships and have that information displayed in various areas.