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:
Can't pick just a few, but I'd go for The One True Source, SSC/ACX and check the top posts.
As a bonus, Scott references LW quite often, and these links are almost universally to high-quality writeups.
Paul Graham's http://www.paulgraham.com/identity.html is a must read and internalize. Almost all of us feel angry when something we hold dear is attacked, and often than not it reveals a part of our identity we didn't know we had.
A valuable sequence to have would be a review of all of Paul Graham's essays. His Twitter - paulg - is also of interest.
You could go by the all-time highest voted:
2020 is in the making: https://www.lesswrong.com/posts/ScqWBr8mxy32PzoDz/2020-review-final-voting
2021 will be next year. The best-of are done one year after when the dust has settled down.
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.