Which book would it be best to start with?
Scalzi's novels generally have a fairly intersting premise or bit of science at its heart, but then fail to do anything interesting with them. Old Man's War and Lock In in particular felt this way to me.
Redshirts is probably his best because it can build off of the reader's knowledge of Star Trek and combine it was some meta commentary. It also has a couple of main characters, which means it minimizes the amount of time with the typical Scalzi main character.
The Scalzi main character is a straight white male old enough to have a college degree, but new to whatever job he has so all the rules can be explained to the reader. He'a an atheist, but will without fail take an opportunity to explain how he is accepting of other beliefs. In Locked In, the main characters takes a couple pages to explain slurs regarding people who suffer from the ficitonal Haden's disease. The person he explains it to apologizes for using offensive terminology and then it never comes up again. The Scalzi main character never makes a mistake either, excepting instances where there was no possible way of knowing, and never does anything morally culpable.
It's always seemed weird to me that Scalzi is the most outspoken SJW on his blog and twitter, but then only writes the most milquetoast protagonists and forgettable sterotypes of a supporting cast.