Whichever you want. It doesn't really matter.

Pick whichever of the orange “starter novels” you want (maybe read series synopsis and pick whichever appeals to you most) and proceed down the flowchart. Once you finish one line, go to another.

Where you start is largely a matter of preference, it's true, but beginning at the start of a subseries makes sense.

Rincewind is a great protagonist. He stars in The Colour of Magic, but several other books also have him. The Color of Magic and The Light Fantastic are a pair, the first two books and one story told in two parts. They're fun but a bit slight. They do introduce Rincewind, Twoflower, The Luggage, Lord Vetinari, The Librarian, Ankh Morpork, Unseen University and a few other things.

The Witches have been my least favorite Discworld books, but I'm much in the minority there. They do have some great characters.

The Death books are uniformly great. Although Death is a main character in all of them, I think it's only Reaper Man in which he's the main character. Other characters are Mort, Ysabelle, Albert and later Susan.

The Watch books center around the watchpeople, but especially Capt Vimes. Other characters include Carrot, Angua, Cotton, Nobby, CMOT Dibbler, Gaspode, Detritus, and Vetinari also usually plays a role.

Many, but not all, of the Discworld novels are focused around the biggest city on the Discworld, Ankh Morpork. The Wizards, Watch and Industrial Revolution series are mostly set there. The Witches novels are mostly set in the country of Lancre, and so are a bit more rural. A few books are what you might call one-offs, set in a place that's never returned to, and that makes them good stand-alone books. I think maybe Small Gods is the best of these.