Favourite Discworld game? Ankh-Morpork, by far. That being said, I've not had much chance to play Clacks all that much yet (just the cooperative variant 2 or 3 times) so it's possible that may prove to be better, assuming we can get it to the table and eventually work out how to play a competitive game as well.
Hmm. Or do you mean Discworld computer game? If so, it's hard to say. They're all quite good. Discworld Noir takes a little time to set up on modern computers, but it's well worth the investment. Discworld II: Mortality Bytes is silly but fun. The first Discworld game is the hardest by far, but also very fun.
Well if you're looking for *Discworld* games for Android, that's going to be tricky. The first two Discworld computer games work with ScummVM which can be installed on Android ( www.scummvm.org ) but you'll still need the original games' data files which can't be obtained legally anywhere at the moment. Unfortunately it doesn't include Discworld Noir, which is one game I'd really like to play again (even on the PC).