For me these have entered into my must haves

• BetterDisplay: For better scaling support for external monitors
• Rectangle: To be able to use a mouse to drag and snap windows
• Pixea: To be able to double click an image with a mouse in any folder and then use arrow keys or scroll wheel to proceed to the next file in the folder. Replaced the stock preview with this.

Something I'm looking for now is the ability to use the forward and back buttons on my mouse when I'm in Finder and want to go back to the previous folder I was in. Doesn't work in Safari either. Works in good old dependable Firefox though.

And separate volume controls for each applications.