Interface
As of Midnight prepatch, you won’t require as many addons as before. The most famous addon Weakauras is not working, BigWigs and Details are no longer mandatory addons since Blizzard developed their own, built into the game. 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 which can one shot you is very important.
Macros
Pop Everything Macro
Pretty useful to just macro everything in 1 bind to avoid spending time in AP to press berserking etc.
#showtooltip
/cast Berserking
Iceblock Cancel Macro
/cancelaura Ice Block
/use Ice Block
Yes in that order is how I have it. Lets you instantly cancel the ice block when spamming the button (faster than GCD). Good for removing DoTs, immune damage last second etc. You can still sit in the block like normally with this macro.
Focus Mouseover Macro
/focus [target=mouseover]
You can focus gateways with this macro and then use a key bind to use the gateway without clicking.
Go into your Settings > Keybinds > Targeting and set a key for "Interact With Mouseover".
Focus Interrupt Macro
#showtooltip
/cast [target=focus, exists, harm, nodead] Counterspell; Counterspell
If you use the focus macro above, you can use this macro to interrupt the focus target.
I recommend having a separate bind for the focus interrupt.