Several of them SHOULD be read in order though, especially the Sam Vimes ones, since he personally goes through more actual development than any other character in the series. From meeting Sybil, to marrying her, to... well, I won't spoil the rest.
And "Mort" should be read before any of the stories featuring Susan, for obvious reasons.
My personal favourite Discworld novel is either "Thief of Time" or "Mort"