It is very difficult to select particular posts or book chapters because many ideas are becoming significant when you have known another ideas already. A context is very important for many ideas.
I can list some posts which seemed very important for me in the past.
Thinking only about posts that made me a better rationalist/thinker/person able to achieve things in the world:
Posts can be from LessWrong, the EA Forum, or any other blog or forum. Articles from other sources are fine as well, as are videos or podcast episodes. Books don't count for the purpose of this post, but if there is a specific chapter in a book that you found particularly useful then that counts.