A Fire Upon the Deep is one of my favourite books ever. It successfully combines hard sf and space opera and doesn't get bogged down in either. It is large like many of its space opera kin but the plotting is such you never feel like you are slogging through fluff. And the technologies and physics are well thought out and their effects on the universe at large are well presented. The aliens in it are probably the most interesting since Niven's Puppeteers.