Two of the best I have seen mentioned, in my opinion, are David Drake's Hammer's Slammers series, and John Ringo's Posleen Series. Drake is dirtier and more in your face with the hardcore reality of combat set in the future, and does a damned good job detailing the human cost to both the winner and loser. Ringo is top notch story telling. I have yet to find anything by him that I didn't like, and I have most of his books. And if you want good action mixed with cutting edge quantum state theory written so any schmoe (me) can understand it, get the books the Ringo did with Travis S. Taylor. Taylor does a good action set, as well. His best is titled Warp Speed, though I pick that arbitrarily (they're all good).
Another outstanding writer is David Weber. He's best known for his Honor Harrington series, which is centered around grand space navy strategy, and the main character's (Harrington) life. He has some great freakin' short series and stand alone stories, as well. In my opinion, he has outdone himself with the latest series of books titled, Off Armagedon Reef, By Heresies Distressed, and By Schism Rent Asunder. There's a fourth out in Hardback, but I'm waiting for paperback.