Harrrrrrd indeed.
I'd say Thud, The Fifth Elephant and Reaper Man.
(Followed closely by Witches Abroad and Going Postal. )
...and Nightwatch
...and Jingo
...and The Truth
Help me!
...and Feet of Clay
and Guards, Guards!
and Pyramids
and Thief of Time
Help me! I can't stop!!! <-- See, three urinating dogs! I'm going nuts.
...and Men at arms
...and The Last Hero
and of course all of the Tiffany Aching stories.
Strangely enough, I'm not so fond of the Rincewind Books anymore, though they are still enjoyable, of course.