What is the best professional military fiction?

Most anything by Tom Clancy. In there is a rich universe of the military environment, down to small details that are correctly depicted. He did hang around a lot of military and had a security clearance as a civilian.

True professional military fiction in my world, W. E. B. Griffin:

CLANDESTINE OPERATIONS, is a thrilling look into the deadly espionage of the Cold War and the beginning of a top secret organization called the Central Intelligence Agency.

THE BROTHERHOOD OF WAR is the first of the Griffin series. Its epic nine novels span the period from World War II to Vietnam, capturing all the drama and challenge of Army life.

THE CORPS series covers the fighting men of the U.S. Marine Corps from World War II to Korea.

The MEN AT WAR series follows the secret agents in Colonel William “Wild Bill” Donovan’s Office of Strategic Services (OSS) during World War II.

The World War II series HONOR BOUND follows OSS agents sent to neutral Argentina to secretly undermine German operations there.

THE PRESIDENTIAL AGENT is a cutting-edge insider’s look at intel and politics in the battle of counterterrorism, from the White House to covert ops in foreign lands.

If historical fiction counts, I would say "Once An Eagle" is among the best I have read.

John Ringo's 4-parter, A Hymn Before Battle, Gust Front, Hell's Faire, and When the Devil Dances is excellent. Essentially, aliens have highly advanced technology, but they haven't had to fight in a LONG time. Then a highly aggressive species emerges and begins to decimate them. They come to earth with a proposal: they will share the technology and build weapons, we supply the soldiers. Great books, some of the best use of power armor in fiction in my humble opinion. There's also some other books in the same universe, including Watch on the Rhine, where the Germans use the alien technology to rejuvenate members of the Waffen SS to fight the invaders.

Starship Troopers.

The book, mind you...not the laughable movies derived VERY loosely from the book.

"The Killer Angels" an ROTC requirement at the University of RI and the basis for the movie, "Gettysburg"

I'm a huge military history buff so I think very highly of the Richard Sharpe series by Bernard Cornwell. The title character is a British soldier in India in 1799 in the first novel called Sharpe's Tiger. He then begins to rise up through the ranks eventually becoming a LTC in Sharpe's Waterloo and then still gong on adventures in retirement in the final book Sharpe's Devil. The author uses historical events and locations and puts his characters into the role of real people. I can appreciate Sharpe because I, too, started as a PVT and am now a MAJ getting ready to retire. They're also really fun reads!

Haven't read any of them, am delinquent in professional reading.

No Once An Eagle? It was fiction written by a Marine. I would have thought a Marine like yourself would have it on his list. It was long book but a quick read as it was great. I saw the movie, forget but I think it was a made for tv movie.