I have switched to using Atom as my main code editor about 2 years ago and never looked back. Atom is open source, cross-platform - built on top of Electron, and can be highly customized.

Here is my Atom setup:

Theme

UI Theme: One Dark
Syntax Theme: One Dark

This is the default theme on fresh Atom installations.
In the past I have been using Atom Dark, and it was quite hard to get used to something new, however One Dark is a lot more pleasing to the eye.

Packages

The Essential

editorconfig
apm install editorconfig

Editorconfig helps developers to maintain consistent coding styles between different editors and platforms. You can also generate a new .editorconfig file using the Command Palette.
Hint: Disable Atom's default "whitespace" package to avoid possible conflicts.

docblockr
apm install docblockr

A powerful package that helps you writing code documentation.
It can generate complete doc blocks, and makes it extremely fast to tab your way through filling-in the variable types and descriptions.
Hint: make sure you check the Settings as it provides some neat customization options.

highlight-selected
apm install highlight-selected

Highlight the current selected word occurrences. Extremely useful for quickly checking variables, functions etc.

file-icons
apm install file-icons

Display file type specific icons everywhere (treeview, tabs, finder etc).
Makes it a lot easier to quickly identifying opened files.

Linter and related plugins

linter
apm install linter

Linter is the base package that provides linting functionality for various file types via plugins. Should be the developer's first choice.

linter-eslint
apm install linter-eslint

Lint JavaScript using eslint
My favorite JS linter at the moment for NodeJS and React projects.
Hint: Check "Disable when no ESLint config file is found" on Settings.

linter-jshint
apm install linter-jshint

Lint Javascript using JSHint
Still need it for some older projects that were setup with it.
Hint: Check "Disable when no jshintrc file in path" on Settings.

linter-jsonlint
apm install linter-jsonlint

Lint JSON files using jsonlint.

linter-php
apm install linter-php

Lint PHP files using the php binary.

linter-pylint
apm install linter-pylint

Lint Python files using pylint.

linter-ruby
apm install linter-ruby

Lint Ruby files using ruby binary.

linter-xmllint
apm install linter-xmllint

Lint XML files using xmllint.

Minimap and related plugins

minimap
apm install minimap

Minimap provides a mini-preview of the full source code on every tab, and comes with a variety of useful plugins.

minimap-cursorline
apm install minimap-cursorline

Display the location of the cursor position.
Hint: you can customize the color via your stylesheet.

minimap-find-and-replace
apm install minimap-find-and-replace

Display the search matches.

minimap-git-diff
apm install minimap-git-diff

Display git diffs, possibly one of the most useful plugins along with the next one.

minimap-highlight-selected
apm install minimap-highlight-selected

Display the current selected word occurrences.

For JavaScript development

js-hyperclick
apm install hyperclick js-hyperclick

It lets you jump to where variables are defined, or even required files.
Hint: there are plugins for other languages based on the hyperclick base package.

autocomplete-paths apm install autocomplete-paths

Provides autocomplete functionality for relative paths and files. Not just for JS of course, but extremely useful on your imports/requires.

Tools

atom-beautify apm install atom-beautify

It can beautify a great variety of files, however I mainly use it to prettify JSON files.

sort-lines
apm install sort-lines

Sort selected lines, or get unique lines. Nice to have.

Other syntax support

atom-jade
apm install atom-jade

Adds Jade template language support.

Competition (VS Code)

You might be wondering why Atom and not VS Code. Especially when VS Code seems a lot faster (have you checked memory consumption though?).

Well at least for my needs, the above packages make me more productive. Some of them are not available on VS Code at all yet, or their respective ones are just not that powerful.

VS Code has some really cool features, like JS debugging with breakpoints that I am using sometimes. The integrated git and terminal support may also appeal a lot users, however I prefer to be using specific apps (like iTerm 2 and GitHub Desktop or SourceTree) that just do a lot more.