Guards! Guards!
Or at least Equal Rites, as an introduction to Granny Weatherwax, or Wyrd Sisters as an introduction to Lancre.
Reason for these:
All these could benefit from good special effects, and we would like a story without Rincewind looking a hundred years older than he really is. Did they follow Kirby's (inaccurate) interpretation?
I like Death, and so does Sky, so Reaper Man may be a good one.
Guards! Guards! may be tricky if produced by Sky because in GP they showed Angua as a watchman and her disappearing would be strange.
But the story is humorous and quite easy to understand in comparison with other watch books. And DRAGOOOONS!!!
I can imagine some awesome music playing as Errol launches from the penhouse.
Doooooo..... Dooooo.... doooooo...... DOOOOODOOOOOOO!!!!!
boom boom boom boom boom boom boom
Great characters as well.
Equal Rites and Wyrd Sisters would introduce the world to Granny Weatherwax, and would make great films because of the adventure sense in Equal Rites and the parodic comedy of Wyrd sisters, which the audience would link with Macbeth.