So here, I will keep track of a list of those "must reads".
Someone asked about must read books. I put books in the Brooklyn Investor book store. Check out the "must read" section there: Investment Must Reads
Berkshire Hathaway (BRK)
Of course, right at the top of the list is Warren Buffett and Berkshire Hathaway. His annual letters to shareholders are absolute must reads for anyone interested in investing. All of them together is better than any book anyone can read about investing or business, and it's all available for free at the Berkshire Hathway website. I can't think of a better expenditure of time for investors than reading all of these, and then even rereading them again after a while.
J.P. Morgan Chase (JPM)
Jamie Dimon probably writes the best letters to shareholders of all CEOs with the exception of Buffett. He is a real straight shooter and hates baloney. His letters are straight, honest and very informative. He takes his time to teach you about the business, what's important, why they do what they do etc... His reports are always worth the read and Buffett has even recommended reading them (and I don't recall Buffett recommending any specific annual report, with the exception maybe of M&T Bank). Here I only recommend the letters written by Jamie Dimon.
The Cumming/Steinberg era annual reports are really good, but the new management also writes in some detail.
Markel has been writing great annual reports for a long time too. They are very good at describing what they do, how they've done, why they will continue to do better etc. They have an impressive track record to go with it. What's great at the MKL website is that all of the letters back to 1986 is available there.
Alleghany also has a great letter. A lot of it is about the macro environment and what they see, but the letters are really well written and worth the read. They also have done pretty well over time.
Fairfax Financial Holdings (FRFHF)
Prem Watsa writes interesting letters too. Recent reports are full of market analysis (how bubbled up they are) and things like that. FRFHF has a great long term track record too; it's worth listening to what such a person has to say about the state of the world.
Loews hasn't been doing too well recently (as of early 2015), but they have a solid long term track record. I like their approach to value investing and always look forward to their annual reports.