Too Long; Didn't Read
I believe ‘composition over inheritance’ is the most important OOP principle understood by very few people writing code in OOP languages and applied by even fewer developers. And I am basing my affirmation on my whole experience as a developer and interviewer at 4 companies since I started writing the first lines of code and getting paid for it back in 2003. To be honest it is also based on the reading that I have done in the past years, and I can also tell you there are a lot of developers better than me that say this.