When he is good. David Drake is without compare. He's best known for his Hammer's Slammers series, which contains most of his best works. A lot of his stories tend to be based on his Viet Nam experiences, which gives them a "No BS" feel to it that makes it stand out from simple fiction. Listed in order of descending quality.
The Forlorn Hope (Not HS) - His Best book
Rolling Hot - If you're good enough, even the dregs of a military force can be very dangerous.
The Warrior - What do you do with someone who's too hard core, for a bunch of ruthless Mercs?
Counting the Cost - Recasting of the Nika Revolt in Constantinople CE 532.
Hammer's Slammers - The Base book. Has seen a number of reprintings with various additional stories added and name changes. Only about a 50% hit rate for the stories, but the ones that do hit, hit BIG!
The Possleen Saga by John Ringo - Very entertaining series. I've not read the last book, but they are listed order in the series.
A Hymn Before Battle
Gust Front
Hell's Faire
When the Devil Dances
Cally's War
Have to second Starship Troopers, the classic. BTW A lot of people seem to think that in this book the requirement of having to have served in the military in order to be a full citizen and vote means that Heinline was a fascist, which is a horrible miss reading of things. His point was that if you want a voice in society, you should have to contribute to the society, which is something I think we could all use more of.
Personally, I never really cared for Armor, which is kind of suprising given my love of mecha and powered armor.
I loved the War against the Cthor, but the series does start to suffer from the main character's manac-depressive self discovery process. As he continually cycles between "Oh, now I understand and I'm no longer a screwed up !@##$" and "I'm a self-centered @#$@E$" states. While they are great books and lot of it does center around the military, I'm not really sure if it should be classified as military fiction. Simply, because it really is so much more about the main character and what he's doing than the military or war.
Of course there are the Classic Dorsai! Books by Gordon R Dickson