All of them must be taken with a grain of salt; the coin market is far more volatile than any "published" guide can keep up with. Furthermore, everyone who publishes a guide has a passing interest in prices being high or, at least, "favorable" to whichever party they serve - Greysheet is favorable to dealers, and may therefore be biased towards "dealer buy" prices.
If you're looking to buy from a dealer, Greysheet is your friend because it will inform you towards what dealers tend to be paying for coins, therefore allowing you to form an opinion of an appropriate price allowing for the dealer to make a little money.
Beyond that, only real-world sales of real-world coins, as evidenced by the larger auction houses and (shudder)
ebay, are true indicators of what a coin is worth
today.