I liked Thud! On the whole, not quite up to the level of Going Postal or Night Watch, but the scenes with Vimes trying to hold it together after the break-in are some of the best and most serious work Pratchett's done.

A Fire Upon the Deep is a great galactic-nerdpocalypse story, though the opening scenes are incomprehensible enough to be rough going. Near the end it has one of the most ingenious Outer Space Big Battels in science fiction.

And you don't need me to tell you what a good book A Confederacy of Dunces is.