I would recommend scrolling down and looking for threads were the reply number is very orange in color. The higher the ratio of likes the post in that thread has, the more orange it is. While not a perfect indicator of what is best to read it does show which threads have received lots of community interest.
Also a lot of the really old threads are outdated because of later source books or errata. Just something to keep in mind.
A few I would suggest:
Hard-to-Find Rules which points out answers to questions that are not easily found or known
Consolidating Possible Expansions of Hermetic Theory covers a massive range of possible expansions to Hermetic Theory, along with talking about how an HR can be classified as such.
Everything in 'Post a Day', as others have suggest.
The Language Table thread from Callon. Great resource for languages in AM.
Don't read "Fan Grimorie?" despite its high post count and like ratio. It is a working thread for a project and without access won't make much sense.