Starting with any of the four on the far left probably won't do you wrong, though.
Small Gods is a much better starter book than Pyramids and is probably amongst the best starting books for Discworld in general. Reaper Man is a better starting Death book than Mort. Wyrd Sisters is a *much* better starting Witches book than Equal Rites. Advising somebody starts with Colour of Magic just seems like a move to drive them away, since it's a much weaker and tonally much different book, that will really only appeal if you've got a good grounding in the fantasy works that it is parodying.

Guards Guards is a good starting book, for what is probably the strongest series overall.