(tl;dr: My essentials, more or less in order; but you’ll have to hold a gun to make me give one up
Keyboard Maestro
Default Folder X
BTT
Karibiner
GeekTool (yep, it still works; you just have to kill its terrible multiple displays “support”)
LingonX
Automator/Script Editor (build your own damn Hazel/Alfred Services and Utilities)
FastScripts (useful for everything you don’t want to remember keystrokes for, even if you do have KM)
Dropbox (you’d be amazed at how much remote control you can perform with Dropbox and scripting)
Carbon Copy Cloner (can’t wait for APFS support a la SuperDuper)
SpamSieve (a geek’s nightmare to setup on multiple accounts, but once it’s running; indispensable)
BBEdit
pCalc
Soulver
CyberDuck or Transmit (best); Forklift is okay
Contacts Sync for Google (if you have to play in that world)
Fantastical (1 is fine; 2 is overkill; simple, fast event/reminder entry with intelligent parsing)
Bartender 2 (not 3; love the shelf; not dev’s fault)
Growl (yep, it still works too; super useful for custom utilities)
Autograph
Clean Slate (or KM)
Sizzling Keys (for the HUD, Find Song and Playlist Selector; otherwise KM)
Acorn or Pixelmator (both fantastic)
iStat Menus (helpful, pretty, but GeekTool and shell can do it all well enough)
A Better Finder Rename (even if you love making your own scripts, this tool is awesome)
MacPilot or TinkerTool (for ease of mods; you can do it all with Terminal)
Pacifist
DiskWarrior
Data Rescue
1Password (legacy, non-subscription; if forced, I can live with Keychain and encrypted files in Dropbox)
(Far too many more to recall right now, but I’d hate my time on a computer without these above; and, yes, the order is pretty much meaningless.)
I can’t believe I got to page seven before someone finally mentioned Keyboard Maestro and Default Folder; these two are crucial to my workflow (been using the former for most of this century; the latter since the 90s), and I tear my hair out when I have to work on a Mac without them.
Mosaic (formerly WindowTidy) has come along way, and the developers have big plans on the way, but there’s not a single window manager today (that I’ve found), including every one listed here, that can properly handle multiple displays (I have six).
However, Keyboard Maestro provides ample tools to make any needed window function; it just lacks portability to configurations that vary wildly; i.e., there’s a lot of time investment for multiple resolutions and arrangements, especially on a per-app basis. But, after decades of experience with AppleScript and KM, I’m doing okay. Adding a grid-based manager that also responds to edge targets and can auto-gutter — across multiple displays — is a real holy grail for me.
BetterTouchTool and Karibiner expands on KM nicely for a very select few mapping features.
I used to use TextExpander, but both their move to subscriptions and poor iOS support/implementation compelled me to absorb its duties into KM and System Keyboard Text Shortcuts. I built a better shortcuts manager for macOS, and now I enjoy universal shortcuts on any device I can login to iCloud, though admittedly, I frequently miss some expansions on iOS.
KM does so very, very, very much more, that it is quite literally the first thing I would have to install if I started from scratch. Thankfully, I’m still running on the same OS X installation and user account dating back to the Public Beta (I truly have prefs files with creation dates of 2001), so Migration Assistant could handle things, but I prefer clone migration to new machines.