I've read them all and I rank Going Postal as the absolute pinnacle of Pratchett's work, with Nation and Small Gods as very close second and third. I'm sure others feel differently, of course, but there's something about Moist's journey from dead man to Postmaster that really, really resonated with me the first time I read it, and continues to this day.
I find the Rincewind series to be the least favorite of mine; I guess because he never seems to grow or learn from his experiences (other than learning to run faster.). However, the hardest I've ever laughed while reading a book is while reading Interesting Times, particularly during Rincewind's run-in with the sumo wrestlers. My roommate at the time thought I was crazy, laughing while reading a book.
For favorite series, I would go with the Guard books, followed by the Tiffany Aching series (as a girl, this was another group of books that really resonated with me.). I also really liked The Truth, but mostly because I was working as a reporter at a small newspaper at the time. That book really captured a lot of the craziness in the newspaper industry.