It’s not that I think A Fire Upon the Deep is perfect, it’s just that it’s got so much in it. There are lots of books that have fascinating universes, and there are lots of first contact novels, and there are lots of stories with alien civilizations and human civilizations and masses of history. The thing that makes A Fire Upon the Deep so great is that is has all these things and more, and it’s integrated into one thrilling story. It has the playful excitement and scope of pulp adventure together with the level of characterisation of a really good literary work, and lots of the best characters are aliens.
It really is the book that has everything. Galaxy spanning civilizations! Thousands of kinds of aliens! Low bandwidth speculation across lightyears! Low tech development of a medieval planet! Female point of view characters! A universe where computation and FTL travel are physically different in different places! An ancient evil from before the dawn of time and a quest to defeat it! A librarian, a hero, two intelligent pot plants, a brother and sister lost among aliens, and a curious mind split between four bodies. And the stakes keep going up and up.