It is probably the best fantasy series ever. And it's certainly the best satirical fantasy series ever.

The best thing is there's just so many of them, you'll have plenty to read, and yet it's not like reading The Wheel of Time or something. Each book starts off a little slow, a little off, but it gathers steam and about 5 pages in it's impossible to put it down.

There are a few movies based on the books too. They're not the greatest, but some of them are fun.

Or, since we have suggestions for both Gaiman and Pratchett, you could just read the book they co-wrote: Good Omens. It's not the best work of either man, but that's like saying "those aren't the best boobs I've ever seen." I mean, they're still boobs right?