This was last asked in 2014 on the annual survey. Since we have strong upvotes, you may vote for 2 posts. Each answer should link to a single post, and I will delete repeats. Please use a strong and weak upvote each for at most one answer (and posting an answer is a vote).
Basically any LessWrong URL is ok - links, events, questions, and archive links to pages only on LessWrong 1.0 are fine. I only expect regular posts and links to be chosen though.
Feel free to comment on answers if you're dying to explain your reasoning, or comment on this post if you're dying to mention your third favorite and lower posts.