This is very tricky - almost like trying to decide which of the eight records you'd save on Desert Island Discs.
But the one book that I re-read most often, aside from Good Omens, which doesn't really count, is Soul Music. Being somewhat connected (by marriage) to a musician, it speaks to me on a lot of different levels. It's also got some of the finest Pratchett puns in it. The one question I've always wanted to ask is whether Terry had the idea for the final 'Elvish' joke and then wrote the book around it, or whether it just seemed to good a joke to not to include.
I'm re-reading UA at the moment, and getting more out of it. When there's a new TP book out, I tend to devour it at one sitting and then leave it for a while before I go back and take my time over reading it again (and again, and again.)
Best wishes,
Tony