Interface
As of Midnight prepatch, you won’t require as many addons as before. The most famous addon, Weakauras, is no longer working; addons like BigWigs and Details are no longer mandatory addons since Blizzard developed their own, built into the game replacements. You can still download numerous addons to skin textures and make certain elements of the UI look better, but like I mentioned earlier, they are not mandatory.
BigWigs - If you prefer boss alerts in the form of bars instead of a timeline, you can download BigWigs to reskin them.
Details! - If you prefer a cleaner look of the damage meters, you can download this addon to reskin it.
Elvui - another UI replacement that overhauls the default Blizzard UI, from unit frames, damage meters, to the Cooldown Manager.
Targeted Spells - In my opinion, a must have for high M+. Having the ability to see whether you are being targeted by a spell or ability in a big pack that can one shot you is very important.
Shimmertracker - addon that puts a simple text countdown in the center of your screen when you do not have any Shimmer/Blink charges.
BetterCooldownManager - As the name suggests, BCDM gives your cooldown manager many more functionalities.
UnhaltedUnitFrames - One of the best unit-frame addons out there. With this addon, you can make your unit frames, including party, raid, boss, personal, and target, look super clean.
Missing Class Buff - Hate missing a buff and not noticing? Dislike forgetting to press Arcane Intellect every time you die? Forgot to drink a flask? Then get this addon, which will remind you every time that you are missing any consumable or buff.
Macros
Blizzard Cursor Macro
This macro removes the green circle you would typically have and lets you cast Blizzard at your cursor, essentially saving you time. If you are using the Blizzard talent, which casts it at your target, then you can skip this macro.
#showtooltip
/cast [@cursor] blizzard
Macro for Stop Casting and Use Ice Block
Pressing this Macro Twice will cancel Ice Block to get out of Block Quickly.
#showtooltip
/stopcasting
/cast Ice Block
/cancelaura Ice Block
Be careful to not Press it twice too fast otherwise you will end up wasting Ice Block.
Macro for Counter Spell Your Focus Target
This Macro is essential for Mythic+. It lets you Counter Spell your Focus Target.
/cast [@focus,harm,nodead][] Counterspell
Mouseover Focus Target Macro
Another Useful Macro to have is a Mouseover Focus Target.
/focus [@mouseover,nodead]
Macro for Mouseover Decurse
If you are tired of having to click separate players in your group to dispel them, then just get this mouseover decurse macro
#showtooltip
/use [@mouseover,help,exists][] Remove Curse