I suspect part of the reason they're doing the books they've done is that they might require less FX/serious costume work, also they are each either the first part of a sub series, or standalone so don't require much prior knowledge of the characters or world (FOC for example would require you to know many of the Watch characters to get full enjoyment, so they'd have to introduce both "old" characters/background, and "new" ones, whilst COM only really needs to introduce half a dozen mainly archetypes in addition to the main pair)
Personally I'd quite like to see a Guards Guards as it's a first in the sub series so it introduces most of the characters pretty much as if you'd never read a discworld book before (no major characters carry over from the other books requiring any real explanations), especially if they got the right actors for the roles of Carrot, Nobbs and Colon (the last pair were the first thing I thought of watching Pirates of the Caribbean).
The downside is that GG would probably need several large sets and a heck of a lot of CGI/FX, not to mention stunts and a fairly large primary cast (from memory the Hogfather adaptation only had about 4 primary cast members.
Reaperman might also be a good choice for an adaptation, certainly I suspect non fans will get the jokes about the supermarket as a predatory beast, and death.
Unfortunately really most of TP's books would really require a lot more than a couple or three hours to do them justice on screen, and a much bigger budget and cast, possibly even a mini series each (with the same actors returning in the same roles where appropriate).
I sometimes think that something like the original HHGTTG tv series idea might be a good way to do it with the narrator possibly giving some of the little "explanations" that TP used to do with the *, **, *** etc
(I've also wondered what a good BBC production of it might be like, if they did it to the same sort of budget as they give some of their major costume drama's).
One final thought that does occur to me, is that Sky seem to be going about doing the Discworld adaptations a little inefficiently. If they did them slightly more in order, and more than one every couple of years, it would probably save them money as a regular series than as "specials" done out of order, as done in some form of order they'd be able to reuse sets, costumes and cast members (not have to risk recasting everyone) rather than having to start from scratch each time (COM sets/costumes would make a start for HF, HF a start for GG, GG for FOC, GP etc).
Mind you that would require a much bigger commitment than Sky have ever shown to any of their own drama's/shows (their own productions tend to be very low budget).
