The best resource to improve your performances, monitor your raid and better understand encounters and fights, Warcraft Logs is an institution we shouldn't need to present anymore.
In our opinion, Wowhead is the most reliable and well formatted source of information. Covering topics as soon as they are live, with a solid work on making the news digestible.
Reddit WoW Competitive
Still in our opinion, the best and most relevant WoW subreddit we know, with week by week new and interesting discussion topics coming up, and newly developed resources like ourselves submitted to the community.
The one place for simming your character. If you don't know what simming is or aren't fully comfortable with it, here's a video from Naguura explaining it.
The biggest collection of Weak Auras, together with different addon profiles ready for import.
One of the finest resources for healers, featuring an equivalent of Raidbots/simcraft for healers, pushing frequent news articles around healing specs going into in-depth theory analysis.
For each spec, Subcreation finds the top 1000 parses and displays for a boss fight or dungeon of your choice, the most used talents, legendaries, items, etc... of that spec.
A good place to analyze your raid's logs of a boss fight. It will highlight mistakes players might have done, and things they could have done better.
If you're starting on a character, or you're not fully comfortable with its rotations, wow analyzer is a great place to start with. We recommend later on once all the lights are turning green to move towards log analysis instead.
Draw a raid layout for each boss that you can share to your guild to explain things. This could become very helpful to raid leaders out there.
Running M+? Want to improve and love to analyze? Keystone Heroes has in-depth analysis for your Mythic+ runs based on Warcraft Logs, including routes, cooldown usage and other improvement vectors.
Gives you the current and upcoming affixes, the reward for each key level, a list of resources for M+ and various data points, all in one page. Great when you're looking for quick info.
Essentially, MDT in your browser. Maybe you want to have your route on your tablet or 2nd display instead of trying to pull it out of your memory during the run?