The Colour of Magic

I gave this only three stars because, while it's a great book, compared with what comes later it's nowhere near the heights the series reaches. If you're new to Discworld, I'd suggest not starting here (other reviewers offer their suggestions) but coming back to it later to see how it all started.
I reread it as I'm trying to reread the series in order, just as I did originally beginning in around 1990. I can't recommend the Discworld novels highly enough, even the least good (like this first entry) are very funny but the best are philosophical masterpieces and comically brilliant. The Colour of Magic suffers from trying to cram too many ideas into a short book - later efforts would have taken just one or two and developed them. It's also more a clear parody of a certain genre of novel, with gods and heroes at the periphery while minor-ish characters like Death would later go on to dominate.
Rereading it now, it feels odd that it was ever published but as you progress through the series you'll be glad it was.