You really could read any of them and understand it, most of the Discworld books work well enough standalone.
But I would heavily recommend reading them according to chronological order, as laid out in the L-Space guide.
One more recommendation (to add to jgaiser's) to hold off on the first two books -- the #1 reason for people getting put off of the Discworld books
permanently (at least, in my experience), is reading
The Color of Magic, and assuming
wrongly that all the other books are like it.
Personally, I would start with
Guards, Guards,
Small Gods,
Going Postal, or
Hogfather.
Okay, I'm breaking my own rule with that last, but meh, it's worth it.
Hogfather is one of the all-time bests. And it's not as connected as the other Death books anyway... (OK, now I'm just trying to convince myself.
)
...
Feel free to do a re-read of the whole series in publication order afterward, if you like...