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Much depends upon what you use your Mac for. Your list will not be the same as mine. Your mileage may vary. Some assembly required (batteries not included).
 
Here are my must have productivity tools:

Typinator - automate text entry and error correction for typos you frequently make. You could script entire paragraphs and more if you frequently type the same things such as replies to emails.
Default Folder X - adds last folder, last file, frequent folders, etc. to menu bar and file dialogue boxes
Alfred - supercharged Spotlight, clipboard, etc. It mirrors some of Typinator's functions as well as 3rd party clipboards.
ForeverSave - saves a copy of the file every time you do a Ctrl-S so you have an archive of past versions in case you mess up and save a file by accident where you have deleted key parts.
Bartender - take control of the menu bar
Capto - set of screen capture tools
1Password - manage passwords, licenses, etc.
Magnet - quickly enlarge window, set to right or left half of screen, etc. with hotkeys
Dropshelf - Drop items into it and move to other windows, such as when you want to add a file to an email.
Carbon Copy Cloner - automate backups, I use it to backup documents to a microSD card on an hourly basis

Others depend on what you want to do.
 
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