Nice list of classics (since I agree with it
), you've got Guards! Guards! twice but that's probably justified. My list would have Small Gods listed twice as well.
Just finished Witches Abroad for the first time, and I'm halfway through Lords and Ladies - the witches are the best!
By the way, Thief of Time for me was a little bit... meh, as was Feet of Clay. Not up to the standard of those classics you've listed. Loved Night Watch, of course. Haven't read Men at Arms, Maskerade, Jingo, Interesting Times, Carpe Jugulum, Fifth Elephant yet so I'll be interested to see what my own personal "classic" list ends up looking like. It seems to be different for everyone.
I suppose part of the problem is none of the Rincewind books are classics? At least in the sense you mean...