My least favourite famous five have to be Camp and Mystery Moor.
There's aspects of both that I love, but I'm just not very keen on them. I do find the setting rather depressing.
I find the atmosphere to be very creepy in both, but I don't think that's entirely it. It's similar in Smugglers' Top, but I quite like that one.
I do find the behaviour of the others towards George to be a bit mean girls in Mystery Moor. They're there really because Anne wants to be there, and George has gone along to keep her company. If I'd been George, I think I'd have left Anne to it well before the boys arrived!
Yes, George has taken an irrational dislike to Henry, but it sounds pretty mutual from their interactions. I'd have expected them to have backed their cousin a bit more rather than laughing it off and inviting Henry to go out with them, which really is a slap in the face for George. They know that George won't go then, so basically they're saying they'd prefer Henry to George.
Maybe Camp was partially effected by my older sister first read it at my gran's, which was next to a train line and told me how scared she was all night (well, probably twice, but it felt like it!)