I've only read one as well, His Majesty's Dragon, so I don't think I can vote. I did really like it though, and I can see why Peter Jackson is keen on making it into a movie. There's some swashbuckling fights on dragonback -- people board dragons like they would a ship on the high seas -- and some amazing aerial battles that just beg for the big screen.
Actually, I need to go read the sequels (although Glasshouse will definitely be on the list now, too -- thanks Alasdair).