Discworld

I have read:
-The colour Magic
-The Light Fantastic
-Sourcery
-Troll Bridge
-Equal Rites

So what Discworld book should I read next?  Should I keep with the Rincewind novels, read the next in the Witches novels, or read something else?  I plan on reading all the books in time but I want to read them in the best order possible so help me out.

It's much better to just read them by publication because you don't get bored of any single character thread. 

Night Watch is also my favourite of the series.

Oh, and the Tiffany Aching books are cute, if you get tired of reading anything that makes you think too much, or just feel like reading something that is certain to have a happy ending.