I keep an living list of stuff that I find useful for M+ for my guild and I figure other people can probably benefit from it too.
Bare Minimum
- Boss Mods (Choose One)
- Good Nameplates [that help you quickly identify priority targets] (Choose One)
- WeakAuras: https://www.curseforge.com/wow/addons/weakauras-2
Good to Have
- OmniCD: https://www.curseforge.com/wow/addons/omnicd
- CDs on Nameplates: https://wago.io/LyVvlKxvs
- M+ Timer: https://wago.io/M+Timer
Affix Specific
- TWW Dungeons: https://wago.io/twwdungeons
Dungeon Specific
- Mists of Tirna Scithe
- Automatic Maze Navigator: https://wago.io/04YHZsxIO
- Maze Helper: https://www.curseforge.com/wow/addons/maze-helper-mists-of-tirna-scithe
Planning & Management
- Raider IO: https://www.curseforge.com/wow/addons/raiderio
- Mythic Dungeon Tools: https://www.curseforge.com/wow/addons/mythic-dungeon-tools
- Standard Pug Routes: https://raider.io/weekly-routes
- Premade Groups Filter: https://www.curseforge.com/wow/addons/premade-groups-filter
- Astral Keys: https://www.curseforge.com/wow/addons/astral-keys
- Angry Keystones: https://www.curseforge.com/wow/addons/angry-keystones