Most feature rich markdown preview among editors: import PDF right inside document, using @import you can run python, it's like having IPython/Jupyter, full math support, full latex (even entire document) support. Tikz, Viz, GnuPlot, UML, ....
Atom is not a native application. As such, performance is subpar and the lag is especially noticeable on larger projects. It also opens a surprising amount of subprocesses and leaks a considerable amount of memory.
UberWriter does away with interface clutter and allows focusing on writing. The only items on the interface are line and character counters and fullscreen and focus mode toggles. Focus mode highlights only the last sentence.
Although configuration is heavily loaded, the starting time of Spacemacs is usually between two and five seconds. Emacs can be run as a daemon though which reduces the client's startup time to a few milliseconds.
MdCharm's interface is designed for productivity. There are tabs for switching between commonly files quickly, shows line numbers, has a side-by-side live preview, supports drag and drop and allows easy access to functionality such as search and replace.