Some of the apps I've started using this year:
Numi - This is a great calculator for Mac. It uses almost-natural language input to do basic arithmetics and conversions. It supports variables, comments, and multiple sheets. It is very simple to use and very Mac-like.
For power users, it also supports Javascript extensions for those who want custom constants, units and functions. Lastly, it integrates with Alfred (putting the Spotlight calculator to shame).
Typora - Great markup editor. As of December 2018, there is no iOS app. So if you require syncing between devices, I would recommend
iA Writer (for a document-centered approach), or
Bear (for a note-centered approach). I use Bear all time, but I still have Typora on my computer for LaTeX equation support.
Fluor - "Fluor is a tool that allows you to automatically change the behavior of the keyboard's
fn keys depending on the active application". This is amazing, especially on a TouchBar MacBook Pro in conjunction with BetterTouchTool. It allows the user to make the standard Function keys default in every application, and automatically change it to the TouchBar's custom actions in just some apps.
IINA - The best video/music player on a Mac. It feels and acts like a native application (even more so than QuickTime player). It supports PiP. The first beta version rolled out a couple of months ago, but the app feels very smooth (much more than VLC) since early alpha versions. The only thing it lacks is AirPlay support. If you can live without it, then this is a perfect application to replace QuickTime and VLC.