3rd party, roughly in descending order of use

Default Folderrapid navigation in open/save dialogs
Al Dente (pro)keeping my battery at 70% – prolonging its life
iStat Menuskeeping me constantly informed about network & CPU activity
BusyCal/Contacssuperior replacement for Calendar and Contacts
CopyClip clipboard history
Command X⌘-x (followed by ⌘-v) in the Finder
Strongboxpassword manager, very Mac-like interface and secure (so far).
Replaced LastPass and 1Password
BBEditall unformatted editing
GraphicConvertergraphics swiss army knife, occasionally helped by Pixelmator
Alfredkeyboard shortcuts, macros across all apps
PDF Expert, UPDFbetter at editing than Apple’s good Preview, but bad pricing models
Reederdaily news on my topics
Houdah SpotSearching → Finding! Much better than Spotlight
PDF Squeezer PDFs much smaller w/o visible loss
ImageOptimperfect image optimiser for jpg, png
VinegarYouTube w/o hassle
SilentKnightdaily check on security (and other) updates
OcenAudioediting & transcoding audio
Jekyllstatic CMS for all websites I author / maintain
Guitar Promusic stuff
iReal Promusic stuff
MacGiroaccessing my bank account securely

Apps from Apple

Mailsince several decades
Safaribrowsing, of course; keep Firefox, Chrome etc. always in dock too
Pageshalf of my (formatted) editing in Pages, especially layouting
Numberssome of my spreadsheet work
Keynotefor (nearly) all my (>900 so far) presentations
Notesquick notes, quick lookup; tried Obsidian
Booksfor reading & audiobook. HORRIBLE interface, getting worse every year
Mapsby now nearly eye-level with Google maps
Musicgood, though annoyingly pushy with ads
Home(too) slowly improving
QuickTime Playerstill my preferred video editor
Time Machinemarkedly improved, slow over WLAN (helped to switch to the 5 GHz band)
Xcodegood for programming

Unrelated – Adam Engst’s great collection of “fn” or 🌐 key shortcuts

🌐-ASelects an item in the Dock, after which you can use the arrow keys to select different items and press Return to switch to the app
🌐-⇧-AOpens Launchpad
🌐-COpens Control Center
🌐-DStarts dictation (or set a modifier key to do this when you press it twice)
🌐-EOpen the emoji picker (same as choosing Edit > Emoji & Symbols)
🌐-FToggles full-screen mode
🌐-HHides current windows to reveal the desktop; a second press restores them
🌐-MSelects the  menu, after which you can use the arrow keys to navigate menus and activate the selected command by pressing Return
🌐-NDisplays Notification Center
🌐-QStarts a new Quick Note in Notes
🌐-⌫Forward delete on keyboards without a Forward Delete key⌦ (or use ^D)
🌐-⏎Equivalent to the Enter key on the numeric keypad, which is slightly different than Return⏎ in certain contexts
🌐 ↑Scroll up one page (same as the Page Up key)
🌐 ↓Scroll down one page (same as the Page Down key)
🌐 ←Scroll to the beginning of a document (same as the Home key)
🌐 →        Scroll to the end of a document (same as the End key)

(Nearly) unrelated…

All glyphs for Apple’s special symbols