As a fan of both HP and Elfman I would have been disappointed if he scored the series. He is too obvious a choice for anything magical, and he's kind of got repetitive over the last few years (although Meet the Robinsons was quite interesting). I'd like to see him branch out of his comfort zone.
I never got the whole Williams phenomenon. His work is grand and I do respect him for what he did when he started out, but I don't believe that he has created a score specially designed for one movie for over 10 years. He makes beautiful main themes, but then the rest is kind of all the same.
I do think Hedwig's theme is quite on cue for HP, and the whole first score is quite good, but the second I did not care for. Three was OK.
Doyle was adequate, but still there was something missing.
So far I like Hooper's approach best, mostly because he was able to take the unbelievably dull and pubescent OotP and create something that was true to the story, yet rich in emotion. I don't feel that usage of simple motives such as Umbridge's theme makes for a worse composition.