Top place would be either 'The School at the Chalet' or 'The Chalet School in Exile'. I can't quite decide between them. Iirc, they're both long books. I love how EBD introduced us to all of the pupils and their families in the first few books. She didn't just create a school - she created an entire community. I think that's what's missing about the books set in Switzerland - we don't get to know the pupils' families in the same way.
I think 'Carola' was the first book I read and I always had a soft spot for it. I also liked 'Three go to the Chalet School' - quite a lot of the book is set outside of the school. In fact, I didn't realize it was a CS book when I started reading it (it was my sister who collected the books and I just picked it off the shelf - obviously the title never registered with me). A lot of readers give out about Mary Lou, but actually, she never wanted to go to school in the first place. And when she did go, she came up with that mad scheme to study like crazy, so she'd end up in the same form as her friend Clem. She's not as perfect as she became in the later books.
Fifth place is a tie between 'The Chalet School and the Lintons' and 'A problem for the Chalet School'. They've similar plotlines.
It's been a few years since I read the books. I wonder what I'd make of them now that I'm middle aged? Even when I read them back in the '80s, there were some things that struck me as odd. For example, in one of the books, a pupil described how her aunt (I think) had escaped the Nazis by disguising herself and putting on make up. Her aunt had never worn make up before and the way her niece described it, make up appeared to be an invention of the devil. Perhaps EBD was insinuating that the woman had disguised herself as a prostitute? In another book, one of the pupils said her family had been living in a tiny, cramped cottage. I was sympathising with her until she said that the cottage only had eight rooms! And I was a bit perplexed by the school's obsession with slang.
I loved Bernhilda's wedding - the drama of the wedding and then her grandmother dying. It was all very satisfying to tween me and I assumed Bernhilda would have a 'happy ever after' life and raise lots of flaxen haired children in that mountain paradise (a bit like Heidi). But of course, these days I'd be shaking my head in despair at how poor Bernhilda never got to have a career and was married off when she'd barely left school.
Grizel was a great character - very complex.
Which book had the story about the Balbini twins and their mother dying? That story made a really big impression on me.
I read the books as I came across them - my sister had a big collection and I also borrowed some from the library. I never got to read them in order, so the action kept switching between Austria, Switzerland and the UK. In some of the books, Joey was still a pupil - in other books she was a wife and mother to assorted children, depending on which book I read . And on that subject, wouldn't it have been unusual for a doctor's wife to have such a big family? I mean, surely Jack knew about the facts of life, even if Joey didn't. Same with Dick and Mollie. I don't think the Russells had quite so many.