I'm a big fan of Gardner, myself, ever since reading his short story "Muffin Explains Teleology To The World At Large", which is a hoot. I liked Expendable a lot, actually, but the other books in the series are good, too. Commitment Hour was a bit of a misstep--it could almost be a Tepper novel. I haven't read his latest, Radiant, yet.

Definitely agree on Moving Pictures, but I really liked Reaper Man. I think my current favourite Discworld book is Thief of Time, though.