They're great, and also some of the strongest ones in the series! After Guards, Guards! the next one is Men at Arms, then Feet of Clay, Jingo, The Fifth Elephant, Night Watch, and Thud!

Or you could go like Werthead said and finish the Death books. Or you could start the Witches with Wyrd Sisters. You can go anywhere, really. It doesn't matter, but you'd probably be confused if you pick up the later Watch or Witches or Death books without having read the ones that come before. But they could all basically stand alone anyway, so it still wouldn't matter. Might get some spoilers though. But there's not really any bad choices!