Order of the Pheonix is definitely my favourite of the Harry Potter series. There is so much wonderful detail of new wizarding places (Grimmauld Place, The Ministry of Magic, St Mungo's Hospital) and some great new characters (Mundungus Fletcher, Kreacher, Mrs Figg, Mrs Black). And then there's the sublime Luna Lovegood, rather like one of Shakespeare's wise fools, with her slanted view of reality and matter-of-fact nonsense, often seeing what others cannot see; she is wise and kind and hilariously funny.
Humour is probably this book's strongest point, the overall plot is not as good as Prisoner of Azkaban, there is too much going on to allow for a neat punchy ending. Nevertheless there is plenty of good drama throughout; the scene where Neville accepts the bubblegum wrappers from his brain-damaged mother is particularly moving, and we get a better insight into adult characters such as Sirius, Snape and Mrs Weasley. Even Fred and George's rebellion is fun - so long as you remember that it's really a kids' book and go with the flow.
Insofar as I can judge, the French translation is flawless; it reads smoothly and does a good job of preserving the flavour and detail of the original, intelligently translating puns, rhymes, comical names and different styles of speech. This is just as much fun to read in French as in English.