Personally I found the witches arc the slowest to get in to. It's too long ago to remember why I found that, but they sat unfinished. I've read them all several times now, and enjoy them, but only after I'd thoroughly got into the rest of Discworld.

The City Watch sub-series is my favourite with the Death arc a close second. laurentl already made recommendation on City Watch, if you want to try the Death arc Mort is the first.