There are too many, even when it comes to series. My favourites series though would be
The Chronicles of Ancient Darkness by Michelle Paver. Excellently written, not unnecessarily long and very enjoyable reads in general.
Seeing as we're talking about the Harry Potter series, I thought I'd chip in. The books I have a very mixed opinion over. While they have an excellent story, good characters and some amusing humour, the writing itself is mediocre. The sense of drama is poorly executed (in particular, JK Rowling never really figured out how to execute tension), and as the books grew darker, they also grew a lot longer largely due to unnecessary plot-stalling filler used mostly to re-explore some of the children-orientated aspects of the series (e.g. two chapters in the fifth book cleaning Sirius' house, which had very little character development and mostly centred around encounters with magical creatures).
Long story short, JK Rowling is a mediocre writer with genius imagination and ideas, combining to make the Harry Potter series a pretty good but unspectacular read.
The film series, however, is well worth watching. Ok, so the first one mostly sucked and the second one was good but could have been better (methinks it had something to do with the director, Chris Colombus, who was renowned for making children's and/or family films as opposed to dramatic cinema, and he ended up making them more kidsy than they needed to be, particularly the first), but from the third one and onwards they were all excellent.
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