Disclaimer: The poster is tipsy right now.
I mentioned in my previous post that I'm not a fan of series.
I though I'd go with middle ground and pick Discworld.
But then the problem of picking the favourite Discworld book pops up.
So I decided to be cheap and just go with the my favourite novel in my favourite Discworld sub-series.
The Colour of Magic (The first book of Rincewind sequence. If I had to pick only one book, it would probably be this one due to the sum of awesome parts.)
Witches Abroad (Witches sequence. I like this one because it has both the humour and a shred of actual philosophy/mysticism. It's also one of the rare moments when we get the glimpse behind Mistress Weatherwax's stern exterior.)
Honourable mention: Maskerade - a Phantom of the Opera parody.
Guards! Guards! (The first book of City Watch sequence. Probably one of the best books of Discworld overall.)
Honourable mention: Thud! - in which Vimes is awesome.
(ETA: Fixed a sentence. ("favourite novel Discworld sub-series - they are one of those I like the best."->"favourite novel in my favourite Discworld sub-series.") My tendency to edit halfway through made a mess again. (It was supposed to be the first novel of every subseries, but then I remembered that I liked some latter entries better than the openers.) Considering that I kept making typos last night, I daresay it turned out very well for a drunken post.)