In reply to Ian Archer:
Generally, it's whichever one I've just finished reading.
I've probably read Lords and Ladies, Reaper Man, and Guards Guards the mostest, possibly with Pyramids in there too.
Of the more recent ones, Night Watch is superb, Thud carries on nicely from the brilliant The Fifth Elephant and...
...if I carry on, I will name almost all of them...
Although I haven't re-read the last one yet. Making Money.