The first one was very much written in an homage to Starship Troopers and it was an early work of his. His very first one was Agent to the Stars (https://en.wikipedia.org/wiki/Agent_to_the_Stars#Publication... - available online at https://scalzi.com/agent/ ) which was a book that he didn't want to write in that he wanted to make sure he could write as a career before writing the book he'd been thinking about for a long time... it's ok, maybe better than ok, but not great. He didn't want to find out that he didn't have the stuff to be a professional writer with the book that he really wanted to write.

That book that he really wanted to write was Old Man's War. I would put it as a good one, but not a great one. A young man inspired by Heinlein and Haldeman writing military science fiction as his first real book.

His later books get better as he became a more mature writer writing new ideas rather than the story he had in the back of his mind for the past decade. The Ghost Brigades (I believe) is better than the previous (it deals with the special forces). The Human Division and The End of All Things were written as serialized chapters that kind of stand on their own and I believe that format worked quite well with him.

I will note that... he likes snark. I don't recall much in his later Old Man's War books... I think he got it out in The Interdependency (a fun series but you have to be ok with snark and casual vulgarity - the Audible is read by Wil Wheaton (some may be turned off by that alone) who is able to read and handle the snark and casual vulgarity quite well). I will recommend it if one is ok with a very snarky character - it's a political intrigue series.