Agree with some of the complaints above: her plots do get very repetitive and her characters tend to be two-dimensional cardboard cutouts. But her vocabulary is by no means limited - a charge levelled at her by many Blyton critics - and she's at her most eloquent and evocative when describing the beauty of the natural world. Her cherry and willow farm stories are lovely. She's also a master at creating atmosphere, particularly in the 'gothier' of her books.
It's for this reason 'The Island of Adventure' and 'Five on Kirrin Island Again' offer two such different reads, albeit their plots are identical. Same with 'The Rilloby Fair Mystery' and 'Five are Together Again'. In both these pairings the Famous Five equivalents are the weakest: likely because it was by far her longest series and she was probably sick of if by the end.
I love, love, love 'The Rilloby Fair Mystery'. A typical locked-door mystery, but I adored the subplot of Snubby making up a silly story about the Green Hands Gang, completely taking in gullible old Uncle Robert but getting steadily creepier as it becomes increasingly and spookily real, seeming to follow him wherever he goes. 'The Rubadub Mystery' which resolves the Barney and father arc, is also a standout of that series.
I'm voting with others upthread for 'The Castle of Aventure', and am not surprised to see it cited as so many people's favourite. Great story of wartime spying and with a really brooding, formidable atmosphere. This book helped instil in me a lifelong love of nature - I love the bit about Jack staying alone in the creepy old castle photographing a family of golden eagles - plus Philip's fox cub. The storm at the end is fabulous and of course it features my favourite Blyton character, Bill Smugs.
I liked the detective series, a departure in genre from her usual adventure stories. They have a humour which is pretty much absent from her other stuff, especially when Fatty takes it into his head to take up ventriloquism ...
Agree with others that the bullying and xenophobia in the school stories doesn't read well as an adult. Of all those books I like Fifth Formers at St Clare's the best. There's a scene of comedic farce when Mamzelle is convinced she's locked up 5 burglars into cupboards in the dead of night - in reality they of course turn out to be girls either up to the usual high jinks or navigating some fairly serious problems.
Yes, there are problems with Blyton's writing and she undoubtedly espoused some awful attitudes even for her time, but my childhood would have been much the poorer without her.