There's no escaping it. I love The Dark o' the Moon. When forced to put my finger on why I prefer it to The Raiders or Silver Sand there is one outstanding reason. Hector Faa.
Strict historians will tell you that the history is compromised, and of course the events cannot fit realistically into the 'actual' timeline of history - Crockett has truncated time to suit his purposes, and the sub-plot can be a bit questionable BUT for me, everything is forgiven for the descriptions of Hector Faa.
He is hero as villain (cut from the same cloth as Heathcliff) and while he may evoke memories of Long John Silver and Captain Hook, he is way beyond pantomime villain, absolutely captivating and the perfect foil to his brother John/Silver Sand.
Add to that the incredible natural description in the novel and it's four hundred pages of sheer pleasure. The Sheil o the Dungeon of Buchan is my favourite place in the world (even if fictional) and the escape in the dark over the Dungeon Range is as exciting an experience as I need to get in fact or fiction.
All I can say is that, if you liked The Raiders, in which Hector has a small but significant part, you should really enjoy The Dark o' The Moon in which he truly comes into his own.
But I'd love to know what other readers think.